Low-Cost Measurement System Using Time Domain Reflectometry

ABSTRACT

A Time Domain Return measurement system for measuring liquid level, linear movement or other measurements which includes a first and second electrode, the second electrode spaced from the first electrode to define a gap, and an electronics assembly connected to the first and second electrodes equipped with a generator for transmitting an electromagnetic signal for propagation along the electrodes. The signal generator has a first analog timing circuit for actuating a slow-rising function of voltage versus time, a second analog timing circuit associated with the first analog timing circuit for actuating a fast-rising function of voltage versus time, and a receive circuit electrically connected to the electrodes, the receive circuit being activated for receiving return echo data associated with the electromagnetic signal transmitted when the fast-rising function is equal or greater than the slow-rising function to determine the position of the second medium with respect to the electrodes.

PRIORITY TO PREVIOUS APPLICATIONS

This non-provisional patent application claims priority to provisionalpatent No. 62/586,160 filed on Nov. 14, 2017. That provisionalapplication was filed by Chester Wildey, but should have included GagikFarmanyan as an inventor. Concurrent with this filing, the inventors areseeking to amend the inventorship on that provisional patent.

BACKGROUND OF THE INVENTION

This invention relates generally to the measurement of one or morematerial properties, and more particularly to an apparatus and methodusing time domain reflectometry (TDR) for determining at least a levelor height of a material within a container, or the position of oneobject with respect to another, and/or a dielectric constant, specificgravity, permittivity, or other property of the material of interest.

Prior art devices that employ time domain reflectometry (TDR) aretypically very expensive and thus not feasible for low-cost devicesrequired for certain products and markets that are cost-competitive. Forexample, known TDR sensors for determining liquid level within acontainer require high-cost high-precision electronic components,including high-precision temperature sensors, capacitors, resistors, andexpensive microcontrollers with very high accuracy timers to determineliquid level with a relatively high degree of accuracy. Such TDR devicesalso employ expensive parts that interface with the measurement probeand the electronics, and can require more assembly and calibration timethan desired, as well as the need for expensive calibration equipmentduring factory calibration, resulting in prohibitive costs that canrarely be justified except where the highest measurement accuracy isrequired.

It would therefore be desirous to provide a TDR measurement system thatovercomes one or more of the disadvantages of prior art solutions. Thiscan be done for example by implementing structures, systems, circuitry,and methods that eliminate or at least substantially reduce expensiveinterface parts, as well as high-cost high-precision electroniccomponents.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a time domainreflectometry (TDR) measurement system for determining a position of amedium to be measured with respect thereto, includes a housing and anelongate measurement probe having a proximal end portion connected tothe housing and a distal end portion. The elongate measurement probeincludes: an outer elongate electrode connected to the housing and beinggenerally cylindrical in shape with at least one inner conductivesurface; an inner elongate electrode being generally cylindrical inshape and located coaxially within the outer elongate electrode. Theinner elongate electrode comprises a first axially extending sectionhaving a first length with a first diameter and a first outer conductivesurface that faces the at least one inner conductive surface; and asecond axially extending section having a second length with a seconddiameter and a second outer conductive surface facing the at least oneinner conductive surface, with the first diameter being smaller than thesecond diameter. A first inner space is located between the outerelectrode and the first axially extending section of the innerelectrode. A second inner space is located between the outer electrodeand the second axially extending section of the inner electrode. A firstspacer is located in the first inner space and has a first spacer borefor receiving the first axially extending section of the inner elongateelectrode, the first spacer being constructed of a material with a firstdielectric constant that, together with the smaller diameter of thefirst outer conductive surface and the at least one inner conductivesurface, describe a first impedance. The second inner space togetherwith the second outer conductive surface and the at least one innerconductive surface describe a second impedance in the absence of themedium to be measured and a third impedance when the medium occupies atleast a portion of the second inner space. An electronics assemblyincludes a transmitter for transmitting an electromagnetic energy pulsealong the elongate measurement probe and a receiver for receiving atleast a return echo from the electromagnetic energy pulse uponencountering a change in the impedance with respect to the inner andouter electrodes to thereby determine the position of the medium locatedin the second inner space. In this manner, the first impedance at leastapproximates the second impedance in the absence of the material to bemeasured to thereby reduce or eliminate a return echo at an interface ofthe first spacer and the second inner space so that the material to bemeasured can be discerned in close proximity to the first spacer.

In accordance with another aspect of the invention, a TDR measurementsystem for determining a position of a medium to be measured withrespect thereto includes a first electrode, a second electrode spacedfrom the first electrode to define a gap between them, with the gapbeing adapted to normally receive a first medium with a first dielectricconstant to define a first impedance and a second medium with a seconddielectric constant displacing the first medium to define a secondimpedance different from the first impedance. An electronics assembly isconnected to the first and second electrodes; the electronics assemblyincludes a pulse generator for creating and transmitting anelectromagnetic energy pulse for propagation along the first and secondelectrodes. The pulse generator has a first analog timing circuit foractuating a slow-rising function of voltage versus time, a second analogtiming circuit operably associated with the first analog timing circuitfor actuating a fast-rising function of voltage versus time, and areceive circuit electrically connected to the electrodes, the receivecircuit being activated for receiving return echo data associated withthe propagated electromagnetic pulse when the fast-rising function isequal to or greater than the slow-rising function to thereby collect themeasurement data and determine the position of the second medium in thegap with respect to the electrodes.

Other aspects, objects and advantages of the invention will becomeapparent upon further study of the following description in conjunctionwith the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the preferred embodiments of thepresent invention will be best understood when considered in conjunctionwith the accompanying drawings, wherein like designations denote likeelements throughout the drawing figures, and wherein:

FIG. 1 is an isometric view of a time-domain-reflectometry (TDR)measurement system in accordance with the invention for measuring liquidlevel, linear position, as well as other material properties, themeasurement system being mounted on a tank and showing for examplevarious levels of material therein as represented by phantom line.

FIG. 2 is an isometric exploded view of the TDR measurement systemincluding a mounting head and elongate measurement probe.

FIG. 3 is an enlarged longitudinal sectional view of the TDR measurementsystem taken along line 3-3 of FIG. 1.

FIG. 3A is an enlarged sectional view of an upper area of the elongatemeasurement probe and mounting head taken along line 3A-3A of FIG. 3,with an upper spacer thereof shown in different cross-hatching fordemonstration of different materials that can be used and for clarity innumbering of the various elements.

FIG. 4 is an isometric view of a time-domain-reflectometry (TDR)measurement system with a mounting head and elongate measurement probein accordance with a further embodiment of the invention mounted on atank having various levels of material therein as represented by phantomline.

FIG. 5 is an isometric exploded view of the second embodiment of the TDRmeasurement system of FIG. 4.

FIG. 6 is an enlarged isometric longitudinal sectional view of thesecond embodiment of the TDR measurement system taken along line 6-6 ofFIG. 4, with fasteners removed for clarity, and showing removablecalibration pins in phantom line associated with the elongatemeasurement probe for calibrating the TDR measurement system inaccordance with an exemplary embodiment of the invention.

FIG. 7 is an enlarged longitudinal sectional view of a TDR measurementsystem in accordance with a further embodiment of the invention with anelongate measurement probe for measuring linear movement between a probebody and plunger of the TDR measurement system.

FIG. 7A is an enlarged sectional view of an upper area of the elongatemeasurement probe and mounting head taken along line 7A-7A of FIG. 7,with an upper spacer thereof shown in different cross-hatching fordemonstration of different materials that can be used and for clarity innumbering of the various elements.

FIG. 8 is an enlarged side elevational view of an exemplary housing ofthe TDR measurement system of FIG. 4 in accordance with the invention.

FIG. 9 is an isometric sectional view taken along line 9-9 of FIG. 8 andshowing inner and outer electrodes of the measurement probe in phantomline.

FIG. 10 is an enlarged top isometric view of an exemplary upper spacerof the TDR measurement system.

FIG. 11 is a sectional view thereof taken along line 11-11 of FIG. 10.

FIG. 12 is an enlarged bottom isometric view of the upper spacer.

FIG. 13 is a sectional view thereof taken along line 13-13 of FIG. 12.

FIG. 14 is an enlarged top isometric view of an exemplary lower spacerof the TDR measurement system.

FIG. 15 is a sectional view thereof taken along line 15-15 of FIG. 14.

FIG. 16 is an enlarged bottom isometric view of the lower spacer.

FIG. 17 is a sectional view thereof taken along line 17-17 of FIG. 16.

FIG. 18A is a top plan view of an electronics assembly having a printedcircuit board (PCB) in accordance with an exemplary embodiment of theinvention and showing exemplary electronic components connected to thePCB and an exemplary connection means for electrically and mechanicallyconnecting the PCB to the elongate measurement probe of the TDRmeasurement system.

FIG. 18B is a top plan view of one of the layers of the PCB inaccordance with an exemplary embodiment of the invention showing a PCBcalibration trace portion of a transmission line and various otherfeatures.

FIG. 19 is a schematic block diagram illustrating the relationshipbetween the electronic circuitry and the measurement probe of the TDRmeasurement system mounted in a tank or container for determining liquidlevel and/or other conditions in accordance with the invention.

FIG. 20 is a block diagram of electronic circuitry of the TDRmeasurement system in accordance with an exemplary embodiment of theinvention.

FIG. 21 is a block diagram of a communication device for receiving anddisplaying measurement data from the TDR measurement system.

FIG. 22 is a schematic of electronic circuitry of the TDR measurementsystem in accordance with an exemplary embodiment of the inventiondivided into modules or sections in broken line to facilitate thedescription thereof.

FIGS. 22A-22J are enlarged schematic diagrams of several of the modulesof the electronic circuitry of FIG. 22.

FIG. 23 is a representative isometric cut-away view of the elongatecoaxial measurement probe electronically sliced in increments of time(or equivalent distance) to illustrate unobstructed current flow betweeninner and outer elongate electrodes and an exemplary method of receivingelectronic measurement data at predefined intervals to thereby constructa measurement curve of the probe including any materials located withinthe coaxial elongate electrodes for determining liquid level or anyother material properties.

FIG. 24 is a diagram of two non-linear equations that form a linearfunction for triggering a series of measurement pulses at the predefinedintervals of time in accordance with an exemplary embodiment of theinvention.

FIG. 25 is a chart illustrating intersections of the non-linearequations at first, second, and third exemplary time intervals to createa linear function in accordance with the invention.

FIG. 26 is a graph illustrating an exemplary measurement curve tracingmethod in accordance with the invention for the TDR measurement system.

FIG. 27 is a graph similar to FIG. 26 illustrating the curve tracingmethod of the invention with a different measurement curve reflective ofdifferent material properties and/or a modified probe configuration.

FIG. 28 is a graph similar to FIG. 26 with the addition of an exemplarycalculated time value trace (TVT) in accordance with the invention thatfollows the measured return echo profile of the TDR measurement systemfor identifying the position of return echoes when the impedance of themeasurement probe changes due to the presence of an anomaly therein.

FIG. 29 is a block diagram illustrating a method for calibrating the PCBcalibration trace in accordance with an exemplary embodiment of theinvention.

FIG. 30 is a block diagram illustrating a method of calibrating themeasurement probe in accordance with one exemplary embodiment of theinvention.

FIG. 31 is a block diagram illustrating a method of calibrating themeasurement probe in accordance with a further exemplary embodiment ofthe invention.

FIG. 32 is a block diagram illustrating a method of determining amaterial level, as well as a position of an object, such as a plunger,within the measurement probe for determining liquid level, materiallevel, or movement of a plunger within the measurement probe inaccordance with exemplary embodiments of the invention.

FIGS. 33-44 are graphs similar to FIG. 28 showing valid intersectionsbetween measured data sets and processor-generated data sets fordetermining the level of diesel within a tank or other container inaccordance with the invention.

It is noted that the drawings are intended to depict only exemplaryembodiments of the invention and therefore should not be considered aslimiting the scope thereof. It is further noted that the drawings maynot necessarily be to scale. The invention will now be described ingreater detail with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and to FIG. 1 in particular, a timedomain reflectometry (TDR) measurement system 10 in accordance with anexemplary embodiment of the invention is illustrated. The TDRmeasurement system 10 is shown schematically connected to the wall 12 ofa tank or container 14 with various levels 16, 18, 20 of material 22located therein, as represented by phantom line. The TDR measurementsystem 10 in accordance with one exemplary embodiment of the inventionincludes a mounting head 24 and an elongate measurement probe 26, whichin accordance with one exemplary embodiment of the invention comprises atransmission line or probe with coaxial conductors that extend from themounting head 24.

An electronics assembly 34 (FIG. 2) is located in the mounting head forpropagating pulses of electromagnetic energy down the elongatemeasurement probe 26, as represented by arrows 15A, 17A, and 19A (FIG.1), which serves as a portion of a waveguide in a guided wave radar(GWR) system, which directs pulsed electromagnetic energy radiationpulses more efficiently than radar measurement systems that propagateradar signals through air.

It will be understood that the term “pulse” as used herein refers to adistinguishable burst, ramp, wave, or other rapid change inelectromagnetic energy, such as a change in amplitude or frequency of asignal imposed on a waveguide or transmission line of the TDRmeasurement system 10. The waveguide or transmission line comprises acalibration trace formed on a PCB and electrodes associated with theelongate measurement probe 26, as will be described in greater detailbelow.

For purposes of the present invention, the pulse can be in the form of aramp-up of energy from a first value, such as a baseline value to ahigher second value, with or without a ramp-down to the first value orother lower value. Likewise, the pulse can be in the form of a ramp-downof energy from a first value, such as a baseline value, to a lowersecond value, with or without a ramp-up to the first value or otherhigher value. Since the propagation of electromagnetic energy will occurat or near the speed of light when air is present between electrodes ofthe elongate measurement probe 26, and perhaps half of that velocity inthe presence of materials to be measured (depending on the dielectricconstants of the materials), in order to increase efficiency, allow theuse of low-cost components, and simpler algorithms for control of thetransmission and reception of the electromagnetic pulse, the wave orpulse of electromagnetic energy ramps up (or down) and does not returnto the baseline value until the end of the elongate measurement probe 26is reached in preparation for a new measurement cycle.

Accordingly, when an electromagnetic energy pulse, burst, ramp, etc.reaches an anomaly, i.e. a dielectric discontinuity in the probe 10 ofsufficient difference to change the impedance of the probe at aparticular location thereof, at least a portion of the electromagneticenergy pulse is reflected back along the waveguide to the electronicssection as a return echo, as represented by arrows 15B, 17B, and 19C,respectively. Characteristics of the return echo depend largely on thetype of anomaly, such as a difference between dielectric properties oftwo materials. Non-limiting examples would include: a) a vapor-to-liquidinterface; b) an interface between two liquids with different densities;c) an interface between a fluid, such as air or liquid and a hardsurface; d) the top of a plunger for distance measurement; or e) thebottom of the probe or other purposely placed feature.

The return echo is received, recorded and analyzed by the electronicsassembly to ultimately determine the location along the probe 26 whereone or more anomalies occurred or is occurring. The anomalous locationcan represent any discontinuity of materials; non-limiting examplesinclude: a) the level of liquid in a tank or container; b) the positionof a rod or plunger with respect to a stationary support; c) theinterface between two liquids (including air or a vapor); d) the levelof granular material within a storage silo; e) the location ofpredefined anomalies such as apertures, thinner or thicker areas; f)spacers or supports at certain positions within the elongate measurementprobe 26; as well as g) anomalies that may occur at one or morelocations in the elongate measurement probe 26, e.g., film build-up onthe measurement surfaces, contaminant deposits, the location of foreignmaterial within the probe 26, etc.

The speed or velocity at which the electromagnetic energy pulse travelsthrough the liquid, solid or gaseous state of different materials canalso be recorded and analyzed to determine other properties of thematerial being measured between the elongate electrodes, such as thedielectric constant, and so on. When the electromagnetic energy pulse orburst comprises a radar signal, the velocity at which the energy pulsetravels through air approaches the speed of light. Depending on thedielectric constant of various materials, the velocity can be slowed toless than half the speed of light, more or less. Accordingly, a veryfast clock pulse is needed.

The TDR measurement system 10 can be associated with stationarycontainers 14 at fixed locations, as well as with transportablecontainers associated with vehicles or the like for measuring one ormore properties of the material located within the container. The TDRmeasurement system 10 can also be associated with linear transducers formeasuring relative position or displacement between two objects.Materials to be measured can be in gaseous, liquid, or solid states.Although the present invention will be described in conjunction withmeasuring the level of liquid within a tank, linear transducers and thedielectric constant of liquids, it will be understood that the inventionis not limited thereto, but may be used for measuring the heights ofseveral different liquids with different densities within a single tank,the dielectric constants of such liquids, as well as the level, height,or other properties of any material that creates a measurable echoduring operation of the TDR measurement system 10.

With particular reference to FIGS. 1-3, the mounting head 24 preferablyincludes a housing 28 and a cover 30 connected to the housing forcreating an interior space or volume 32. The housing 28 and cover 30 canbe constructed of any suitable material; known suitable materialsinclude metals, plastics, ceramics, composites, elastomers, andcombinations thereof, but these elements are not limited to thosematerials; nor do they need to be made of the same material. Inaccordance with a preferred embodiment of the invention, the housing 28comprises an electrically conductive material, such as aluminum orstainless steel, for electrically connecting to the elongate measurementprobe 26. An electronics assembly 34 (FIG. 2) is located within theinterior space 32. A gasket 36 is sandwiched between an upper surface 38of the housing 28 and a lower surface 40 of the cover 30 to seal theinterior space 32 from the outside environment. The gasket 36 ispreferably annular in shape and ensures a proper seal between thehousing 28 and the cover 30.

A plurality of apertures 42 extend through the cover 30 and acorresponding number of apertures 44 extend through the gasket 36 forreceiving threaded fasteners 46. The fasteners in turn thread intoapertures 48 formed in the upper surface 38 of the housing 28 to therebyseal the interior space 32 when the fasteners are tightened. Preferably,the threaded apertures 48 are formed outside of the interior space andlocated at spaced positions around the periphery of the interior space32. It will be understood that the housing 28 and cover 30 can beconnected via other connecting means, including but not limited tocooperating snap-fit engagement members on the housing and cover,press-fitting the housing and cover together, adhesive bonding, welding,mechanical fastening, and so on.

The housing 28 has a first mounting portion 50 located outside of thetank or container 14 and a second mounting portion 52 that extends fromthe first mounting portion and into the tank. The first and secondmounting portions of the housing 28 are preferably integrally formed asa single unit during manufacturing. However, it will be understood thatthe mounting portions can be formed separately and connected togetherusing well-known connecting means without departing from the spirit andscope of the invention.

The first mounting portion 50 is generally cylindrical in shape andincludes the upper surface 38 for receiving the cover 30, a lower wall54 adapted for contacting the wall 12 of a container 14 when connectedthereto, and a continuous side wall 56 that extends between the uppersurface 38 and lower wall 54. The interior space or volume 32 is formedin the upper surface 38 and bounded by the side wall 56 and lower wall54.

As best shown in FIG. 2, the interior space is generally square-shapedor rectangular-shaped for receiving a printed circuit board (PCB) 58 ofthe electronics assembly 34. A wiring harness 60 is located in theinterior space 32 and extends through an opening 62 formed in the sidewall 56. The harness 60 includes three electrically conductive wires64A, 64B, 64C (FIG. 3) insulated from each other and located within asheath 66. The wires 64A, 64B, and 64C are in turn electricallyconnected to the PCB 58 through well-known connection means and extendoutwardly from the first mounting portion 50 through the opening 62 inthe side wall 56. A terminal 72, comprising a receptacle or plug, isconnected to the wires 64A, 64B, and 64C via termination pins (notshown) in a well-known manner. The receptacle or plug 72 is adapted forconnection to a mating plug or receptacle, respectively (not shown)associated with a vehicle, tank, system, or machine (not shown). One ofthe wires 64A, 64B, and 64C is electrically connected to a power supply75 (FIG. 20), such as the power supply of a vehicle, external battery,line power, and so on, for example, for providing electrical power tothe electronics assembly 34 (FIG. 2) and related components. Likewise,another of the wires is electrically connected to ground on the PCB 58or other grounded component of the measurement system or vehicle, systemor machine associated therewith. Another of the wires is electricallyconnected to a microcomputer 83 (FIG. 20), processing circuitry, or thelike, associated with the electronics assembly 34, for providing signalsto a remote location indicative of one or more measured properties ofthe material being measured by the elongate measurement probe 26, asdetermined by the electronics assembly 34.

The transmission of signals related to the measured properties can bevia the receptacle or plug 72 and wiring harness 60 to a hard-wireddisplay 77 (FIG. 20) associated with the TDR measurement system,vehicle, machine, system, etc. Signals can also, or alternatively, besent wirelessly via a radio-frequency (RF) transceiver 79 (FIG. 20) toan independent external display 81 (FIG. 21) associated with a vehicle,machine, system, a portable device such as a smartphone, tablet,computer, and so on, in a well-known manner. The signals can beindicative of one or more conditions inside the tank or container 14(FIG. 1), e.g., liquid level, material level, specific gravity, liquidor material type, vapor space, temperature, pressure, density, orambient conditions outside of the tank, e.g., temperature, humidity,atmospheric pressure, and vehicle tilt.

Although discussion of the present invention is predominantly related toliquid level measurement within a tank and associated properties, itwill be understood that any gas, vapor, liquid or material property thatcan be measured or otherwise determined using the system and method ofthe present invention falls within the spirit and scope of theinvention.

Moreover, although three electrical wires are schematically shown forproviding power, ground, and signal, it will be understood that the TDRmeasurement system 10 can comprise more or fewer electrical wires orconductors depending on the information transmitted and the remotedevice, machine, or system that receives and displays the materialinformation.

A grommet 70 or the like is mounted in the opening 62 with the cable 60extending therethrough. The grommet 70 presses against both the opening62 and the cable 60 to both seal the interior space 32 and providestrain relief for the cable 60. Opposing nuts 71 and 73 are threadedonto the grommet 70 on either side of the side wall 56 for securing thegrommet 70 to the first mounting portion 50.

A plurality of mounting holes 74 (FIG. 2) extend through the firstmounting portion 50 from the lower wall 54 to the upper surface 38. Themounting holes 74 are larger than the apertures 48 previously described,and are also located at spaced positions around the periphery of theinterior space 32 for receiving mounting studs 76. The mounting studs 76are typically mounted on the wall 12 of the tank 14 and surround anopening 80 (FIG. 3) formed in the tank wall 12 through which the secondmounting portion 52 extends. Mounting holes 78 are formed in the gasket36 and correspond in size and position to the mounting holes 74.Likewise, corresponding mounting holes 82 are formed in the cover 30.When the TDR measurement system 10 is mounted to a tank 14 or the like,the mounting holes 74, 78, and 82 of the mounting head 24 are alignedwith the studs 76 of the tank. The mounting head of the TDR measurementsystem 10 is then pressed or fitted onto the studs 76 until the lowersurface 54 of the first mounting portion 50 is adjacent the tank wall 14and the second mounting portion 52 is located in the tank opening 80 andextends into the tank. In this position, the studs 76 protrude outwardlyfrom the cover 30. Nuts 84 (FIG. 3) or the like can then be threadedonto the studs 76 to secure the TDR measurement system 10 to the tankwall.

It will be understood that the means for mounting the TDR measurementsystem 10 to a container or the like is given by way of example only,and can vary without departing from the spirit and scope of theinvention. For example, the tank may be provided with threaded openingsfor receiving bolts or other fasteners that extend the oppositedirection through the mounting openings of the mounting head 24. Inaddition, many tanks have either a straight threaded opening or NPTthreaded opening for receiving a liquid level transducer or the like.The type of threaded opening depends on the material, or liquid storedin the tank, and in order to accommodate such arrangements,corresponding threads (not shown) can be provided on the first mountingportion 50 and/or the second mounting portion 52 or other portion of theTDR measurement system 10 for mating with the threaded tank opening (notshown). Other known means for connecting the measurement system to atank, container, wall, or the like can also be used without departingfrom the spirit and scope of the invention.

With particular reference to FIGS. 2 and 3, the second mounting portion52 of the housing 28 is generally cylindrical in shape and includes acontinuous annular side wall 86 with a first wall section 87 thatextends from the lower wall 54 of the mounting head and through theopening 80 formed in the wall 12 of the tank 14. The first wall section87 forms a generally cylindrical inner space or volume 89 (FIG. 3)bordered by an inner surface 85 for receiving the elongate measurementprobe 26. The housing 28, and thus the inner surface 85, is preferablyelectrically conductive so that the second mounting portion 52 of thehousing 28 becomes a longitudinal extension of the elongate measurementprobe 26. The annular side wall 86 also includes an upper extension 91(FIG. 3) that extends into the interior space 32 above the lower wall 54and has an upper annular surface 88 for supporting the PCB 58 of theelectronics assembly 34. The surface 88 is also preferably constructedof an electrically conductive material to form an electrical connectionbetween the housing 28 and the PCB 58, and thus an electrical connectionbetween the PCB 58 and an elongate outer electrode 90 of the measurementprobe 26 via the housing 28. An opening 103 (FIG. 3A) is locatedcentrally in the upper extension 91 of the annular side wall 86 and iscircumscribed by a circular inner ledge 101 (best shown in FIG. 3A)formed in the upper extension 91. The opening 103 communicates with afirst annular space or volume 121 between the inner and outerelectrodes, and the inner space 89 of the first wall section 87.

The elongate measurement probe 26, in accordance with an exemplaryembodiment of the invention, comprises a coaxial transmission linehaving the first, or outer, elongate electrode 90 and a second, orinner, elongate electrode 92 spaced from, and coaxial with, the outerelongate electrode 90. As shown, the outer elongate electrode 90comprises an outer hollow electrically conductive cylinder or tube 94.The inner elongate electrode 92 comprises an inner solid electricallyconductive rod 96 that extends coaxially inside the outer elongateelectrode 90 to form a coaxial-style transmission line for the elongatemeasurement probe 26. The coaxial transmission line can be used formeasuring the impedance of whatever may be located in an annular innermeasurement space or volume 98 (FIG. 3) formed between the innerconductive surface 97 of the outer elongate electrode 90 and the outerconductive surface 99 of the inner elongate electrode 92.

Although the elongate measurement probe 26 is described herein as acoaxial conductor or transmission line, it will be understood that theelectrodes can be of any suitable shape or size and spaced at anysuitable distance so long as one or more properties or conditions ofliquid or other material or medium located in a space between theelectrodes can be measured or determined utilizing the system andmethods of the present invention. Depending on the type of liquid orother medium being measured, a thin, insulative coating can be appliedto one or both electrodes to both protect the electrodes from corrosionand finely adjust the nominal impedance value (NIV) by adjusting thethickness of the insulative coating, the surface area, and the distancebetween the outer and inner electrodes.

The outer elongate electrode 90 is received and secured in the annularside wall 86 by press-fitting as one preferred method of assembly. Tothat end, the outer elongate electrode 90 can include a knurled section95 or the like formed at or near an upper edge or face 93 of the outerelongate electrode 90 for biting into the inner conductive surface 85 ofthe annular side wall 86 during assembly, so that the side wall 86becomes an extension of the outer elongate electrode 90.

However, it will be understood that the outer elongate electrode can beconnected to the housing 28 through other well-known connection means,such as mechanical fastening as shown in FIG. 2 for example, where athreaded fastener 123 engages a threaded opening 125 formed in thesecond mounting portion 52 of the mounting head 24 for engaging thesurface 95 of the outer elongate electrode 90 to hold the elongateelectrode 90 in the second mounting portion 52 through frictionalengagement with the fastener. Other means for connecting the outerelectrode 90 to the housing 28 can include, but is not limited to,welding, adhesive bonding, clamping, snap-fit engagement, threading,heat-shrinking, and so on. In accordance with a further embodiment ofthe invention, the outer elongate electrode 90 can be integrally formedwith the annular side wall 86.

No matter what connection means is used, the outer elongate electrode 90is preferably in electrical contact with the inner conductive surface 85of the annular side wall 86, which is in turn electrically connected toground associated with the PCB 58 and/or the wall 14 of the tank 12 orother grounding location associated with the TDR measurement system 10,the tank 12, and/or the machine or system associated with the tank. Itwill be understood that electrical ground of the TDR measurement system10 can be electrically connected to, or isolated from, the electricalground of the tank and/or machine or system associated with the tankwithout departing from the spirit and scope of the invention.

As shown in FIGS. 3, 18A and 18B, the PCB 58 is multi-layered in thecurrent embodiment and includes an upper surface 111 (FIG. 3), a lowersurface 113, and at least one intermediate surface 119 located betweenthe upper and lower surfaces, with first and second conducting openingsor thru-holes 102 and 104, respectively (FIG. 18) extending through thethickness of the PCB 58 between the upper and lower surfaces thereof.The PCB 58 is mechanically and electrically connected to the annularside wall 86 by a pair of fasteners, such as self-locking screws 100(FIG. 18A), that extend through the first and second conductive openingsor thru-holes 102, 104 (FIG. 18B) of the PCB 58 and thread into threadedopenings (not shown) formed in the upper annular surface 88 of theannular side wall 86 (shown in phantom line in FIG. 18A). A circulartrace 105 can be formed on the PCB 58 for surrounding each conductiveopening 102, 104 for electrical contact with either a head 107 of thefasteners 100 or a self-locking washer 109 associated with thefasteners. In this manner, the PCB is both mechanically and electricallyconnected to the annular side wall 86 of the housing 28. The innerconductive surface 85 of the side wall 86 is in turn mechanically andelectrically connected to the outer elongate electrode 90, preferablythrough press-fitting, so that the PCB 58 is electrically connected tothe outer electrode 90 via the annular side wall 86.

Preferably, the outer elongate electrode 90 is connected to ground onthe PCB 58 via the annular side wall 86. Although it is preferred thatthe mounting head be constructed of electrically conductive material,such as stainless steel, aluminum, and the like, it will be understoodthat the mounting head can be constructed of electrically insulatingmaterial and provided with conductive surfaces through well-knownsurface treatment techniques, without departing from the spirit andscope of the invention. It will be further understood that the mountinghead can be completely non-conductive, and the measurement probe can beelectrically connected to the electronics assembly 34 without themounting head acting as an intermediate conductor between themeasurement probe and the PCB 58.

The inner elongate electrode 92 is mechanically and electricallyconnected to the PCB 58 via a self-locking screw 106 (FIG. 18A) thatextends through a third conductive opening or thru-hole 108 (FIGS. 3A,18B) of the PCB 58 and threads into a threaded opening 110 (FIG. 3A)formed in a top surface 112 (FIGS. 2, 3A) of the inner elongateelectrode 92 (shown in phantom line in FIG. 18A). The upper end 114 ofthe inner elongate electrode 92 has a step 115 (FIGS. 2, 3A) thatdefines a reduced section 116 with an outer conductive surface 117 thatis also cylindrical in shape to ensure that the impedance as measuredbetween the inner and outer elongate electrodes remains substantiallythe same with the addition of an upper spacer 120 (FIG. 3), therebyeliminating or at least minimizing a return echo from electricalelectromagnetic energy pulses transmitted along the coaxial measurementprobe. With this construction, the TDR measurement system is notsubjected to large return echo signals at the interface between the PCBand the elongate electrodes, and is therefore capable of measuringlevels or heights of liquids or other materials in close proximity tothe upper spacer 120 so long as the return echo associated with the topsurface of the liquid or other material is greater than any return echothat may be generated at the interface, thereby increasing themeasurement range and accuracy of the actual level or height of materialin the elongate measurement probe.

Preferably, the third conductive opening 108 (FIG. 18B) is centeredbetween the first and second conductive openings 102 and 104,respectively, to ensure that the inner elongate electrode 92 is coaxialwith the outer elongate electrode 90. The PCB openings 102, 104 arepreferably connected to electrical ground of the PCB while the PCBopening 108 is connected to other electronics and electronic circuitryfor sending electromagnetic energy pulses down the inner elongateelectrode 92 and receiving data reflective of the electromagnetic energypulse at predetermined locations along the length of the elongatemeasurement probe via the outer elongate electrode 90 or vice-versa,including return echoes due to one or more anomalies along the PCBand/or measurement probe that causes a measurable change in impedance,such as the interface between two or more materials having differentdielectric constants, such as air and liquid within the measurementprobe, the interface between two liquids, a probe marker signifying thebeginning and/or end of the elongate measurement probe, calibrationmarkers, and so on.

A first upper spacer 120 is annular in shape and located in a firstinner annular space or volume 121 located between the outer conductivesurface 117 of the inner electrode 92 and the inner conductive surface85 of the annular side wall 86 of the housing 28, and between the upperedge or face 93 of the outer elongate electrode 90 and the lower surface113 (FIG. 3A) of the PCB 58. The upper spacer 120 is sandwiched betweenthe upper edge or face 93 (FIG. 2) of the outer elongate electrode 90and the ledge or flange 101 that extends radially inwardly from theinner surface 85 (FIG. 3). The upper spacer 120 also includes a portionthat extends beyond the flange 101 so that the upper spacer abuts, or isin close proximity to, the PCB 58 to minimize changes in impedanceduring operation.

Although not shown, a resilient layer can be provided between the PCB 58and the upper spacer 120 to ensure a snug fit of the upper spacer withinthe first inner annular space 121 while accommodating manufacturingtolerances. As with the first upper spacer 120, the resilient layer canhave features and/or be constructed of one or more materials with one ormore dielectric constants and features that are intended to minimize anychanges in impedance during operation.

A central bore 124 extends through the upper spacer 120 for receivingthe reduced cylindrical section 116 of the inner elongate electrode 92.The upper spacer 120 also includes an outer annular groove 126 formed inan outer side surface 127 for receiving an outer O-ring or seal 128 toseal the upper spacer 120 against the inner surface 85 of the annularside wall 86. An inner annular groove 130 (FIG. 3) is formed in a lowersurface 134 of the upper spacer 120 and intersects with the central bore124 for receiving an inner O-ring or seal 132 to thereby seal the upperspacer 120 against the reduced section 116 of the inner elongateelectrode 92.

The upper spacer 120 can include other features, such as a bottomannular groove 136 (FIG. 3) formed in the lower surface 134 of the upperspacer, and a second inner annular groove 138 formed within the upperspacer and intersecting the bore 124, to minimize or eliminate ameasurable change in the impedance, due to the outer O-ring 128, as theelectrical electromagnetic energy pulse transitions between theelectronics assembly and the upper spacer 120, and between the upperspacer 120 and the second annular space or gap 98 (FIG. 3) formedbetween the inner conductive surface 97 of the outer elongate electrode90 and the outer conductive surface 99 of the inner elongate electrode92. The annular gap, in the absence of liquid or other material beingmeasured, is normally filled with air or other material(s) in thegaseous phase. For a liquid level transducer, material above the liquidcan be in a gaseous state, for example, when a single liquid level isbeing measured. In addition, the material above the liquid can be in aliquid state for measuring the level of two or more immiscible liquids.As an example, it may be desirable to measure the level of both dieselfuel and water that may be located in a fuel tank. Likewise, it iswithin the purview of the invention to measure the level or height ofmaterials having different dielectric constants, as well as measuringthe dielectric constants of known or unknown materials based on thevelocity of the electrical electromagnetic energy pulse travelingthrough the material(s) being measured.

The default reference material and phase of that material between theelongate electrodes will largely determine the nominal impedance value(NIV) of the elongate measurement probe that is used as a reference atany particular location along the probe length absent any anomalies thatmay occur to change that value, such as the presence of liquids, solids,powders, and so on. Accordingly, the upper spacer 120 is preferablyformed with various features, along with the reduced cylindrical section116 and O-ring material, to thereby ensure that the NIV of the upperspacer 120 and related features approximates the NIV of the measurementprobe in the absence of measurable materials and material states, tothereby substantially reduce or eliminate any return echo from such ananomaly, and allow the measurement of liquid or material height in closeproximity to the upper spacer 120.

In accordance with one aspect of the invention, the NIV can rangebetween above 0 (zero) Ohms and below 377 Ohms for the elongatemeasurement probe 26 coaxial transmission line. The upper limit is theimpedance of free space, and therefore it is expected that the TDRmeasurement system 10 of the invention would operate below that level.However, in order to facilitate development, testing, and calibration ofthe TDR measurement system 10, an NIV of 50 Ohms has been selected byway of example and practicality, since this value is the standardtransmission line impedance for RF devices as well as the testingequipment for such devices. Since the majority of RF test equipmentemploys a nominal impedance of 50 Ohms, the test equipment can bedirectly connected to the electronics assembly of the TDR measurementsystem without the need for impedance transformation adaptors duringdevelopment, testing, and calibration.

With an NIV of 50 Ohms, the upper spacer 120 is preferably constructedof one or more materials that approximate(s) the impedance of air (orother material within the space between the electrodes). When it isanticipated that air will normally be located within the space 98between the elongate electrodes, a suitable acetal resin, such aspolyoxymethylene or acetal homopolymers, sold under the trade nameDelrin™ by DuPont, for example, can be used along with the variousfeatures described above to ensure that the average NIV is sufficientlyconstant, to minimize and/or eliminate any echoes that may occur alongthe length or height of the upper spacer 120.

It will be understood that the upper spacer 120 can be constructed ofother materials or a combination of materials without departing from thespirit and scope of the invention, so long as any echo caused by theupper spacer 120 is sufficiently small to ensure that echoes caused bydifferent materials being measured between the electrodes can berecognized, even in close proximity to the upper spacer. By way ofexample, when it is desirous to measure liquid level within a tank orcontainer, the use of a material for the upper spacer 120 that minimizesor eliminates a return echo caused by the upper spacer and anycomponents, such as the O-rings, connected to the upper spacer, ensuresthat a return echo caused by an upper surface of the liquid proximal tothe upper spacer 120 can be recognized. In this manner, the total probelength or height for measuring liquid level is maximized, whilemanufacturing costs are significantly reduced over prior art solutions.

Although the preferred embodiment of the upper spacer 120 substantiallyreduces or eliminates a return echo so that further return echoes arenot rejected or overpowered by a return echo at the upper end of themeasurement probe 26, in some applications, it may be desirable tocreate an anomaly, such as a change in impedance caused by a change inmaterial construction at or along the upper spacer 120 that will in turngenerate a return echo having a repeatable signature, when it isdesirous, for example, to calibrate a distance between the PCB-innerelectrode transition and a point along the length of the upper spacer,or locate the bottom of the upper spacer or any other axial positionalong the upper spacer for calibration of the measurement system,marking a particular point along the elongate measurement probe, and soon.

It will be further understood that a particular NIV or range of NIV'scan be used without departing from the spirit and scope of theinvention. For example, the use of 33 Ohms as the nominal impedancevalue of the elongate measurement probe 26 allows the greatest powerhandling capability, while the use of 75 Ohms as the nominal impedancevalue results in the least amount of signal loss. Accordingly, theparticular nominal impedance value can greatly vary without departingfrom the spirit and scope of the invention.

As shown in FIG. 3, a second, or lower, spacer 140 is located in thesecond inner annular space 98 of the elongate measurement probe 26 at alower end 141 thereof between the inner surface 97 of the outerelectrode 90 and the outer surface 99 of the inner electrode 92. Inaccordance with one embodiment of the invention, the lower spacer 140 isconstructed of a conductive or semi-conductive material and configuredto create a short across the inner and outer electrodes to therebyproduce a large anomaly and thus a return echo with a large negativeslope during operation to signify the end of the measurable length ofthe probe 26.

In accordance with another embodiment of the invention, the lower spacer140 is constructed of an insulative material and configured to isolatethe inner and outer electrodes, to thereby produce a large anomaly andthus a return echo with a large positive slope during operation tosignify the end of the probe.

In accordance with yet a further embodiment of the invention, the lowerspacer 140 is constructed of a material similar to the upper spacer andconfigured to minimize or eliminate any anomalies and thus minimize oreliminate the creation of any return echo that might signify the end ofthe measurement probe. In accordance with another embodiment of theinvention, the lower spacer 140 is constructed of a semi-conductivematerial and configuration to create a small anomaly and thus a smallreturn echo with a small positive or negative return echo to signify theend of the measurement probe. With this last embodiment, measurement ofliquid or material level in close proximity to the lower end of themeasurement probe can be realized without interference from a largerend-of-probe return echo.

The lower spacer 140 includes an annular body 142 with an annularopening 144 that extends axially through the annular body so liquid orother material from a container 14 (FIG. 1) can enter the second innerannular space 98 for monitoring liquid level and/or other properties bythe TDR measurement system 10. Apertures 146 (FIG. 3) located at theupper end of the measurement probe extend through the outer elongateelectrode 90 and the annular wall 86 to allow air or other material inthe gaseous phase within the second inner annular space 98 to vent intothe container 14 as fluid fills the inner annular space through theopening 144.

It will be understood that the lower spacer 140 can be configured manydifferent ways to accommodate a particular material to be measured, adesired return echo profile or elimination of a return echo at the lowerend of the measurement probe. It will be further understood that thelower spacer 140 can be eliminated without departing from the spirit andscope of the invention, so long as the inner and outer electrodes areadequately supported.

To that end, as shown in phantom line in FIG. 3, one or moreintermediate spacers 150 can be positioned at different locations alongthe second inner annular space or volume 98 of the measurement probe 26to add support to the inner and outer electrodes. In accordance with oneembodiment of the invention, each intermediate spacer 150 provides apredetermined impedance value different from the impedance value whenthe electrodes are in air or liquid. Since the different impedance valuewill result in an echo of a predetermined profile (depending on thematerial being measured in the inner annular space 98), this can be usedto further calibrate the probe since the echo will occur at the knowninstalled location of the intermediate spacer 150, thereby ensuring thatthe probe is always calibrated during operation. Each intermediatespacer 150, in accordance with a further embodiment of the invention, isarranged so that the impedance across the annular space does notsignificantly change to thereby minimize or eliminate any echoes thatmay occur because of the intermediate spacer 150.

With reference now to FIGS. 4-6, a second embodiment of the TDRmeasurement system 210 is further illustrated, which is somewhat similarto the TDR measurement system 10 previously described, with fasteningelements being removed in FIGS. 4 and 6 for clarity. The TDR measurementsystem 10 is shown schematically connected to the wall 212 (FIG. 4) of atank or container 214 (shown in phantom line) with one or more levels216, 218, 220 of material 222 located therein, as represented by phantomlines. The TDR measurement system 210 in accordance with one exemplaryembodiment of the invention includes a mounting head 224 and an elongatemeasurement probe 226, which in accordance with one exemplary embodimentof the invention, comprises a transmission line or probe with coaxialconductors that extend from the mounting head 224.

An electronics assembly 234 (FIG. 5) is located in the mounting head forsending pulses of electromagnetic energy down the elongate measurementprobe 226, as represented by arrows 215A, 217A, and 219A for example.The electronics assemblies 34 and 234 are similar in construction andtherefore will be described with respect to the electronics assembly234. When an electromagnetic energy pulse is transmitted through theelectronics assembly 234 and down the elongate measurement probe 226,one or more anomalies that change(s) the impedance of the probe at aparticular location within the space between the elongate electrodesalong the length of the probe, causes at least a portion of thetransmitted electromagnetic energy pulse to be reflected back to theelectronics section as a return echo, as represented by arrows 215B,217B, and 219B, respectively, and a portion of the energy step or pulsemay continue to propagate down the length of the elongate measurementprobe 226. Characteristics of the return echo depend largely on the typeof anomaly, such as an air/liquid interface, a vapor/liquid interface, aliquid-liquid interface, an air/solid interface, a liquid/solidinterface, and a solid/solid interface, each with different densities ordielectric properties to thereby create a return echo for determining alocation or position of the interface(s). The return echo is thenrecorded and analyzed by the electronics assembly to ultimatelydetermine the location along the probe 26 where one or more anomaliesoccurred or is occurring. The location can represent for example, thelevel of liquid in a tank or container, the position of a rod or plungerwith respect to a stationary support, the interface between variousmaterials and their gaseous, liquid and/or solid phases, as well as thelevel of granular material within a storage silo, and so on.

The speed or velocity at which the electromagnetic energy pulse travelsthrough the liquid, solid or gaseous phase(s) of different materials canalso be recorded and analyzed to determine other properties of thematerial being measured between the elongate electrodes, such as thedielectric constant, and so on.

As with the TDR measurement system 10, this second embodiment can beassociated with stationary containers 214 at fixed locations, as well aswith transportable containers associated with vehicles or the like formeasuring one or more properties of the material located within thecontainer. The TDR measurement system 210 can also be associated withlinear transducers (FIG. 7) for measuring relative position and/ordisplacement between two objects, as will be described below. As in theprevious embodiment, the material(s) to be measured can be in gaseous,liquid, and/or solid phase(s), and can be used for measuring the levelof liquid within a tank, relative displacement between objects, thedielectric constant of liquids, locations of anomalies along the lengthof the transducer 210, measuring the heights of several differentliquids with different densities within a single tank, the dielectricconstants of such liquids, as well as the level, height, or otherproperties of any material and/or material interface that creates ameasurable echo during operation of the TDR measurement system 210.

With particular reference to FIGS. 5, 6, and 7-9, the mounting head 224includes a housing 228 and a cover 230 connected to the housing forcreating an interior space or volume 232. The housing 228 and/or cover230 can be constructed of any suitable material as aforementioned withrespect to the first embodiment. In accordance with a preferredembodiment of the invention, the housing 228 comprises an electricallyconductive material, such as aluminum or stainless steel, for directelectrical connection to the elongate measurement probe 226. Anelectronics assembly 234 (FIG. 5), similar to the electronics assembly34 previously described, is located within the interior space 232. Agasket 236 with a central opening 237 (FIGS. 5, 7) is sandwiched betweenan upper surface 238 of the housing 228 and a lower surface 240 of thecover 230 to seal the interior space 232 from the outside environment.The gasket 236 is preferably annular in shape and constructed of anelastomeric material, felt, cork, or other known materials that exhibitresiliency and compressibility for ensuring a proper seal between thehousing 228 and the cover 230.

A plurality of apertures 242 extend through the cover 230 and acorresponding number of apertures 244 extend through the gasket 236 forreceiving threaded fasteners 246 (FIG. 5). The fasteners 246 are in turnthreaded into apertures 248 formed in the upper surface 238 of thehousing 228 to thereby seal the interior space 232 when the fastenersare tightened. Preferably, the threaded apertures 248 are formed outsideof the interior space and located at spaced positions around theperiphery of the interior space 232. Each threaded aperture 248 isbordered by an annular wall or rim 249 (FIG. 9) that extends upwardlyfrom the upper surface 238. An annular wall or rim 247 (FIG. 8) bordersthe upper wall 238 and is at the same height as the annular walls 249.The wall 249 and rim 247 rest against the lower surface 240 of the cover230 to ensure proper alignment of the gasket 236 with the housing 228and to provide a predetermined distance between the lower surface 240 ofthe cover 230 and the upper surface 238 of the housing 228. In thismanner, the cover and housing will always be consistently spaced duringassembly and limit compression of the gasket 236. It will be understoodthat the housing 228 and cover 230 can be connected via other connectingmeans, including but not limited to cooperating snap-fit engagementmembers on the housing and cover, press-fitting the housing and covertogether, adhesive bonding, welding or brazing, mechanical clamping,fastening, and so on.

The housing 228 has a first mounting portion 250 located outside of thetank or container 214 (FIG. 4) and a second mounting portion 252 thatextends from the first mounting portion and into the tank. The first andsecond mounting portions 250, 252 of the housing 228 are preferablyintegrally formed as a single unit during manufacturing. However, itwill be understood that the mounting portions can be formed separatelyand connected together using well-known connecting means withoutdeparting from the spirit and scope of the invention.

The first mounting portion 250 is generally cylindrical in shape andincludes the upper surface 238 for receiving the cover 230, a lower wall254 adapted for contacting the wall 212 of a container 214 (FIG. 4) whenconnected thereto, and a continuous side wall 256 that extends betweenthe upper surface 238 and lower wall 254. The interior space or volume232 is bounded by the side wall 256 and lower wall 254.

As best shown in FIG. 5, the interior space is generally configured forreceiving a printed circuit board (PCB) 58 of the electronics assembly234. Electric wires (not shown) are located in the interior space 232and extend between the PCB 58 and a receptacle 260 extending into anopening 262 formed in the side wall 256. The receptacle 260 has a baseplate 263 with a first projection 265 extending forwardly therefrom anda second projection 267 extending rearwardly therefrom and into theopening 262 in the side wall 256. The forward projection includes apredetermined pinout of electrical contact pins (not shown) inelectrical communication with the PCB 58 via the electric wires (notshown) in a well-known manner for receiving a complementary plug (notshown) associated with a wiring harness (not shown) of a vehicle,system, or the like, to communicate information from the TDR measurementsystem 210 and provide electrical power thereto. The base plate 263 fitswithin a square-shaped mounting frame 277 integrally formed with andextending from the side wall 256. A gasket 271 similar in shape to thebase plate 263 and mounting frame 277 is sandwiched between a flatsection 279 of the side wall 256 within the frame 277 and the base plate263. For mounting the receptacle 260 to the housing 228, fasteners 269extend through mounting apertures 281 in the base plate 263,corresponding mounting apertures 283 in the gasket 271, and thread intothreaded mounting apertures 273 (FIG. 8) formed in the mounting frame277. It will be understood that the receptacle 260, gasket 271, andmounting frame 277 are not limited to the square-shaped configuration asdescribed and shown, but can be of any suitable shape or configurationto accommodate a large variety of wiring harnesses and/or electricalplugs. Accordingly, a wiring harness (not shown) is connected to a powersupply 75 (FIG. 20), such as the power supply of a vehicle, externalbattery, line power, and so on, for example, for providing electricalpower to the electronics assembly 234 (FIG. 5) and related components.The wiring harness is also connected to ground on the PCB and/or othergrounded component, as well as signal out, representing one or moremeasured properties of the liquid, air or vapor space, solid material,and so on, being measured by the elongate measurement probe 226 and/ordetermined by the electronics assembly 234. As in the previousembodiment, the electronics assembly 234 is also capable of transmittingsignals to a hard-wired display 77 (FIG. 20) associated with thetransducer 210, vehicle, machine, system, etc.

Referring now to FIG. 21, signals can also, or alternatively, be sentwirelessly via a radio-frequency (RF) transceiver 79 (FIG. 20) to anindependent external display 456 (FIG. 21) associated with a userinterface means 450, including a portable device (FIG. 21), such as avehicle, machine, system, a remote smart device such as a smartphone,tablet, laptop computer, as well as stationary devices such as desktopcomputers, generators and other machinery, and so forth. The signals canbe indicative of one or more conditions inside the tank or container 14(FIG. 1) including liquid level, material level, specific gravity,liquid or material type, vapor space, temperature, pressure, density,and so on, ambient conditions outside of the tank such as temperature,humidity, atmospheric pressure, vehicle tilt, and so on, as well asother conditions and so forth. The RF transceivers 79, 454 can compriseany suitable device or combination of devices transmitting and/orreceiving signals representative of the one or more conditions,information related to tank geometry including strapping information andlook-up tables (for liquid level measurement), information for theparticular liquid to be measured in the tank, program updates andinstructions to the TDR measurement system, and so on.

When the remote device 450 is embodied as a smartphone or tablet thatcan communicate via Bluetooth™ technology and/or other wirelessfrequencies, information from the TDR measurement system can beassociated with a user-specific application (app) downloaded onto thesmart device. The smart device can include a user input 458 in the formof a touch-screen, push-button switches responsive to touch or pressure,a wireless or hardwired keyboard, and so on, to select informationrelated to the TDR measurement system and/or the liquid level, materialcondition, other material properties, as well as other functions,including but not limited to, wirelessly coupling with the TDRmeasurement system, selecting information to be displayed on bothdevices, selecting information to be transmitted to the TDR measurementsystem for programming the transducer for a particular tank geometry,when the transducer is configured to measure liquid level, liquidproperties, look-up tables, and so on, for use by the processor 83. Thepower supply 460 is connected to the processor 452 and transceiver 454and can include one or more rechargeable batteries so that the smartdevice has portability.

As best shown in FIGS. 4, 5 and 9, a plurality of mounting holes 274extend through the first mounting portion 250 from the lower wall 254 tothe upper surface 238. Each mounting hole 274 is bordered by an annularwall or rim 275 that extends upwardly from the upper surface 238 and isat the same height as the annular rims 247 and 249 to ensure properalignment of the gasket 236 with the housing 228 and to provide apredetermined distance between the lower surface 240 of the cover 230and the upper surface 238 of the housing 228, as previously described.

The mounting holes 274 are larger than the apertures 248, and are alsolocated at spaced positions around the periphery of the interior space232 for receiving mounting studs 76 (such as shown in FIGS. 1 and 3)that are typically mounted on the wall 212 of the tank 214 and surroundan opening 280 (FIG. 7) formed in the tank wall 212 through which thesecond mounting portion 252 extends. Mounting holes 278 (FIG. 5) areformed in the gasket 236 and correspond in size and position to themounting holes 274. Likewise, corresponding mounting holes 282 areformed in the cover 230. When the TDR measurement system 210 is mountedto a tank 214 or the like, the mounting holes 274, 278, and 282 of themounting head 224 are aligned with the studs 76 (FIG. 1) of the tank.The mounting head of the TDR measurement system 210 is then pressed ontoor fitted over the studs 76 until the lower surface 254 of the firstmounting portion 250 is adjacent the tank wall 214 and the secondmounting portion 252 is located in the tank opening 280 and extends intothe tank. In this position, the studs 76 protrude outwardly from thecover 230. Nuts 84 (FIG. 3) or the like can then be threaded onto thestuds 76 to secure the TDR measurement system 210 to the tank wall.

It will be understood that the means for mounting the TDR measurementsystem 210 to a container or the like is given by way of example only,and can vary without departing from the spirit and scope of theinvention. For example, the tank may be provided with threaded openingsfor receiving bolts or other fasteners that extend the oppositedirection through the mounting openings of the mounting head 224. Inaddition, many tanks have either a straight threaded opening or NPTthreaded opening for receiving a liquid level transducer or the like.The type of threaded opening depends on the material, or liquid storedin the tank, and in order to accommodate such arrangements,corresponding threads (not shown) can be provided on the first mountingportion 250 and/or the second mounting portion 252 or other portion ofthe TDR measurement system 210 for mating with the threaded tank opening(not shown). Other known means for connecting the transducer to a tank,container, wall, or the like can also be used without departing from thespirit and scope of the invention.

With particular reference to FIGS. 7-9, the second mounting portion 252of the housing 228 is generally of frusto-conical shape and includes anannular side wall 286 with a first wall section 287 that extends fromthe lower wall 254 of the mounting head and through the opening 280formed in the wall 212 of the tank 214, and a second wall section 295that extends upwardly into the interior space 232 from the lower wall254. The first wall section 287 has an inner surface 285 that forms agenerally cylindrical inner space or volume 289 (FIG. 9) for receivingthe elongate measurement probe 226 (FIG. 7). The first wall section 287also has an outer surface 291 that slopes inwardly toward a central axis293 of the cylindrical inner space 289 to complement the shape of theopening 280 in the wall 212 of the tank 214.

The second wall section has an upper annular surface 288 for supportingthe PCB 58 of the electronics assembly 234. The housing 228, and thusthe inner surface 285, is preferably electrically conductive so that thesecond mounting portion 252 of the housing 228 becomes an extension ofthe elongate measurement probe 226. The surface 288 is also preferablyelectrically conductive to form an electrical connection between thehousing 228 and the PCB 58, and thus an electrical connection betweenthe PCB 58 and an elongate outer electrode 290 (shown in phantom line inFIG. 9) of the measurement probe 226 (FIG. 6) via the housing 228.

The elongate measurement probe 226, in accordance with an exemplaryembodiment of the invention, comprises a coaxial transmission linehaving a first, or outer, elongate electrode 290 and a second, or inner,elongate electrode 292 spaced from, and coaxial with, the outer elongateelectrode 290. The outer elongate electrode 290 comprises an outerhollow electrically conductive cylinder or tube 294 with a conductiveinner surface 297. The inner elongate electrode 292 comprises an innersolid electrically conductive rod 296 with an outer conductive surface299 that extends coaxially inside the outer elongate electrode 290 toform the coaxial transmission line. The coaxial transmission line can beused for measuring the impedance of whatever may be located in anannular inner measurement space or volume 298 (FIGS. 6, 7) formedbetween the inner conductive surface 297 of the outer elongate electrode290 and the outer conductive surface 299 of the inner elongate electrode292.

Although the elongate measurement probe 226 is described herein as acoaxial conductor, it will be understood that the electrodes can be ofany suitable shape and/or size and spaced at any suitable distance solong as one or more properties and/or conditions of liquid or othermaterial or medium located in a space between the electrodes can bemeasured and/or determined utilizing the system and/or method(s) of thepresent invention. Depending on the type of liquid or other medium beingmeasured, a thin, insulative coating can be applied to one or bothelectrodes 290, 292 to both protect the electrodes from corrosion andfinely tune the NIV by adjusting the thickness of the insulativecoating, the surface area, and the distance between the outer and innerelectrodes.

The outer elongate electrode 290 is received and secured in the innerannular measurement space 289 by friction-fitting the outer surface 301(FIG. 6) of the tube 294 with the inner surface 285 of the of the firstwall section 286 for example. To that end, the outer elongate electrode290 can include a knurled section 303 (FIG. 5) or the like formed at ornear an upper edge or face 305 of the outer elongate electrode 290 forbiting into the inner conductive surface 285 of the annular side wall286 during assembly, so that the side wall 286 becomes an electricallyconductive extension of the outer elongate electrode 290.

However, it will be understood that the outer elongate electrode can beconnected to the housing 228 through other well-known connection means,such as welding, adhesive bonding, mechanical fastening, threading,heat-shrinking, and so on. In accordance with a further embodiment ofthe invention, the outer elongate electrode 290 can be integrally formedwith the annular side wall 286.

No matter what connection means is used, the outer elongate electrode290 is preferably in electrical contact with the inner conductivesurface 285 of the annular side wall 286, which is in turn electricallyconnected to ground associated with the PCB 58 and/or the wall 214 ofthe tank 212 or other grounding location associated with the TDRmeasurement system 210, the tank 212, and/or the machine or systemassociated with the tank or TDR measurement system 210. It will beunderstood that electrical ground of the transducer 210 can beelectrically connected to, or isolated from, the electrical ground ofthe tank and/or machine, frame, or system without departing from thespirit and scope of the invention.

As in the previous embodiment, the PCB 58 is mechanically andelectrically connected to the annular side wall 286 by a pair offasteners, such as self-locking screws 100 (FIG. 18A), that extendthrough first and second conductive openings or thru-holes 102, 104(FIG. 18B) of the PCB 58 and thread into threaded openings (not shown)formed in the upper annular surface 288 of the annular side wall 286(shown in phantom line in FIG. 18A). A circular trace 105 can surroundeach conductive opening 102, 104 for electrical contact with either ahead 107 of the fasteners 100 or a self-locking washer 109 associatedwith the fasteners. Likewise, a circular trace 118 (FIG. 18B) cansurround the conductive opening 108 for electrical contact with either ahead 107 of the fastener 106 or a self-locking washer 109. In thismanner, the PCB is both electrically and mechanically connected to theannular side wall 286 of the housing 228 and the inner electrode 92,292. The inner conductive surface 285 of the side wall 286 is in turnmechanically and electrically connected to the outer elongate electrode290, preferably through press-fitting, so that the PCB 58 iselectrically connected to the outer electrode 290 via the annular sidewall 286.

In accordance with a further embodiment of the invention, the outerelectrode 90, 290 can have a wall thickness or flange wide enough toreceive the fastener 100 or other connection means for direct electricaland mechanical connection to the PCB, thereby bypassing the annular sidewall 86, 286 of the housing 28, 228.

Preferably, the outer elongate electrode 290 is connected to ground onthe PCB 58 via the annular side wall 286. Although it is preferred thatthe mounting head be constructed of electrically conductive material,such as stainless steel, aluminum, brass, and so on, it will beunderstood that the mounting head can be constructed of electricallyinsulating material and provided with conductive surfaces throughwell-known surface treatment techniques, without departing from thespirit and scope of the invention.

As in the previous embodiment, the inner elongate electrode 292 ismechanically and electrically connected to the PCB 58 via theself-locking screw 106 (FIG. 18A) that extends through a thirdconductive opening or thru-hole 108 (FIG. 18B) of the PCB 58 and threadsinto a threaded opening 302 (FIGS. 5-7) formed in a top surface 312(FIGS. 5, 9) of the inner elongate electrode 292.

The upper end 314 of the inner elongate electrode 292 has a step 315that defines a reduced section 316 with an outer conductive surface 317that is also cylindrical in shape to ensure that the impedance asmeasured between the inner and outer elongate electrodes remainssubstantially the same with the addition of an upper spacer 320 (FIG. 7)with a dielectric constant that is different than the dielectricconstant of the air, vapor or other fluid in the inner annularmeasurement space 298, thereby eliminating or at least minimizing areturn echo from electrical electromagnetic energy pulses transmittedalong the coaxial measurement probe as the electromagnetic energy rampor pulse propagates through the transition area between the upper spacer320 and the inner measurement space 298. With this construction, the TDRmeasurement system is capable of measuring levels or heights of liquidsor other materials in close proximity to the upper spacer 320, therebyincreasing the measurement range and accuracy of the actual level orheight of material in the elongate measurement probe.

With the third conductive opening 108 (FIG. 18B) being centered betweenthe first and second conductive openings 102 and 104, respectively, theinner elongate electrode 292 is coaxial with the outer elongateelectrode 290. As in the previous embodiment, the PCB openings 102, 104are connected to electrical ground of the PCB while the PCB opening 108is connected to other electronics and electronic circuitry for sendingelectromagnetic energy ramps or pulses down the inner elongate electrode292 and receiving data reflective of the electromagnetic energy pulse atpredetermined locations along the length of the elongate measurementprobe via the outer elongate electrode 290 or vice-versa, includingreturn echoes due to one or more anomalies along the PCB and/ormeasurement probe, such as the interface between air and liquid withinthe measurement probe, the interface between two liquids havingdifferent dielectric constants, a probe marker signifying the beginningand/or end of the elongate measurement probe, and so on.

Although the invention is described in terms of mechanical fasteners forultimately electrically and mechanically connecting the outer and innerelectrodes to the PCB, it will be understood that other connection meanscan be used, including but not limited to, adhesive bonding withconductive adhesive, soldering, brazing, surface welding, and so on.

As best shown in FIGS. 5, 7, and 9-13, the upper spacer 320 is annularin shape and located in a first inner annular space or volume 321 (bestshown in FIG. 9) which is a portion of the cylindrical inner space orvolume 289 previously described. The first inner annular space 321 islocated between the outer conductive surface 317 of the reduced section316 of the inner electrode 292 and the inner conductive surface 285 ofthe annular side wall 286 of the housing 228, and between an upper edge305 of the outer elongate electrode 290 and an annular flange 307 (FIG.9) of the upper annular surface 288. The annular flange 307circumscribes an opening 309 located centrally in the upper extension295 of the annular side wall 86 and intersects with the first innerannular space 321. The flange 307 functions as a stop member for theupper spacer 320 during assembly. The upper spacer 320 is sandwichedbetween the upper edge or face 305 of the outer elongate electrode 290and the flange 307.

A resilient, circular-shaped layer or sheet 323 is provided between thePCB 58 and the upper spacer 320 to ensure a snug fit of the upper spacerwithin the first inner annular space 321 to eliminate any air gaps orempty spaces that may occur between the upper spacer 320 and the PCB 58due to manufacturing tolerances, small differences in orientationbetween the upper spacer 320 and the PCB 58 that may occur duringassembly, and so on. In this manner, relatively large echoes that mightotherwise occur in the air gaps or empty spaces are eliminated. Acentral bore 325 extends through the layer 323 for receiving the reducedcylindrical section 316 of the inner elongate electrode 292. As with thefirst upper spacer 320, the resilient layer 323 can comprise one or morematerials having one or more dielectric constants and/or features tominimize changes in impedance during operation, as well as one or morematerials and/or layers of materials with different resiliencies.

A central bore 324 extends through the upper spacer 320 between a lowersurface 334 and an upper surface 340 thereof for receiving the reducedcylindrical section 316 of the inner elongate electrode 292. The upperspacer 320 also includes an outer side surface 327 that extends betweenthe lower surface 334 and upper surface 334, with an outer annulargroove 326 formed therein for receiving an outer O-ring or seal 328 tothereby seal the upper spacer 320 against the inner surface 285 of theannular side wall 286. An inner annular groove 330 (FIGS. 12 and 13) isformed in the lower surface 334 of the upper spacer 320 and intersectswith the central bore 324 for receiving an inner O-ring or seal 332(FIGS. 5 and 7) to thereby seal the upper spacer 320 against the reducedsection 316 of the inner elongate electrode.

The O-rings 328 and 332 can be constructed of any suitable resilientand/or flexible material capable of sealing the upper spacer 320 withthe annular side wall 286. Likewise, the resilient sheet or layer 323can be constructed of any suitable resilient and/or flexible materialthat seals against the upper spacer 320 and the PCB 58. Such materialscan include, but are not limited to, rubbers, elastomers, silicones,fluorocarbons, compounds and/or combinations thereof, and so on. Theselection of a particular material for the O-rings and resilient layer323 will depend on the material selection for the upper spacer 320 andany impedance-modifying features to thereby create a reference nominalimpedance value that at least approximates the NIV of the inner annularmeasurement space or volume 298 in the presence of air or vapor, i.e. inthe absence of material to be measured.

The upper spacer 320 can include other features, such as a lower annulargroove 336 (FIGS. 12 and 13) formed in the lower surface 334 of theupper spacer, and an upper annular groove 338 (FIGS. 10 and 11) formedin the upper surface 340 of the spacer 320. The lower and upper annulargrooves have a diameter, depth, and width that serve to minimize oreliminate a measurable change in the impedance as the electromagneticenergy ramp, burst or pulse transitions between the electronics assemblyand the upper spacer 320, and between the upper spacer 320 and the innerannular measurement space or volume 298 (FIG. 7) between the innersurface 297 of the outer elongate electrode 290 and the outer surface299 of the inner elongate electrode 292. As shown in FIGS. 11 and 13,each annular groove 336 and 338 have a different diameter, but aresimilar in width and in depth. Preferably, the depth of each annulargroove is approximately half the thickness of the upper spacer 320, asmeasured between the upper surface 340 and lower surface 334. In thismanner, the combination of the dielectric constants of the upper spacermaterial, the air or vapor in the empty grooves, the reduced diameter ofthe inner electrode, as well as other impedance modifying features,ensure that changes in impedance from the reference nominal value in theannular space 298 are minimized in the area occupied by the upper spacer320 to thereby minimize or eliminate return echoes during measurement.

The second annular measurement space or volume 298 (FIG. 6) formedbetween the inner surface 297 of the outer elongate electrode 290 andthe outer surface 299 of the inner elongate electrode 292, in theabsence of liquid or other material being measured, is normally filledwith air or the gas(es) present in the atmosphere within which theelongate measurement probe 226 is immersed or located. For a liquidlevel transducer, the annular measurement space 298 can be filled with acombination of liquid and gas. The liquid normally would fill up theannular measurement space 298 from a lower end of the measurement probe226 with air and/or a gaseous phase of the liquid filling in the annularmeasurement space 298 above the liquid.

An upper opening 335 and a lower opening 337 (FIG. 6) can be formed inthe outer electrode 290 at upper and lower ends of the elongatemeasurement probe 226 for permitting the ingress and egress of liquidand/or gas into the inner annular measurement space 298 from a space orvolume associated with a container, tank, or the like. The upper opening335 serves to purge the annular measurement space 298 of gas as theprobe fills up with liquid. The openings 335 and 337 can also be usedfor calibration purposes by inserting conductive or semi-conductive pins339 (shown in phantom line) into the openings for example, to short theelongate electrodes 290, 292 or otherwise vary the impedance of theelongate measurement probe 226. With the distance between thecalibration pins 339 being known, calibration and/or recalibration ofthe elongate measurement probe 226 can be obtained both at the place ofmanufacture and in the field by comparing a measured electronic distancebetween the pins 339 (dependent on the initial or calibrated time pulsesbetween return echoes at the calibration pin locations) and the actualphysical distance between the calibration pins, then adjusting systemclock timing pulses to correspond with the actual physical measurement.In this manner, the TDR measurement system can be calibrated to a highdegree of accuracy without the need for expensive calibration equipment.A method of calibrating the TDR measurement system will be described ingreater detail below.

As with the first embodiment, it is within the purview of the inventionto allow the measurement of two or more immiscible liquids, such as thelevel of both diesel fuel and water that may be located in a fuel tank.Likewise, the present invention can measure the level or height ofmaterials having different dielectric constants, measuring thedielectric constants of materials based on the velocity of theelectrical electromagnetic energy pulse traveling through thematerial(s) being measured, as well as linear movement between twoobjects, as will be described in greater detail below.

The default reference material and phase of that material (such as airfor example) within the inner measurement space between the elongateelectrodes will largely determine the nominal impedance value (NIV) ofthe elongate measurement probe used as a reference against any anomaliesthat may occur to disturb that value, such as the presence of liquids,solids, powders, and so on. Accordingly, the upper spacer 320 ispreferably formed with various features, along with the reducedcylindrical section 316 and O-ring material, to thereby ensure that theNIV of the upper spacer 320 and related features approximates the NIV ofthe measurement probe in the absence of measurable materials andmaterial states, to thereby substantially reduce or eliminate any returnecho from such an anomaly, and allow the measurement of liquid ormaterial height in close proximity to the upper spacer 320. The rangesof suitable nominal impedance values as discussed above are alsoapplicable to the present embodiment and therefore will not be furtherelaborated on.

It will be understood that the upper spacer 320 can be constructed ofother materials or a combination of materials without departing from thespirit and scope of the invention, so long as any echo caused by theupper spacer 320, the O-ring material, and other impedance-modifyingfeatures, is sufficiently small to ensure that echoes caused bydifferent materials being measured between the electrodes in the innerannular measurement space 298 can be recognized, even in close proximityto the upper spacer 320. By way of example, when it is desirous tomeasure liquid level within a tank or container, the use of a materialfor the upper spacer 320 that minimizes or eliminates a return echocaused by the upper spacer and any components, such as the O-rings,connected to the upper spacer, ensures that a return echo caused by anupper surface of the liquid proximal to the upper spacer 320 can berecognized. In this manner, the amount of probe length for measuringliquid level is maximized, while manufacturing costs associated with thetransition area between the electronics assembly and the elongateelectrodes, including the upper spacer 320, O-rings, and other impedancemodifying features, are significantly reduced over prior art solutions.

Although the preferred embodiment of the upper spacer 320 substantiallyreduces or eliminates a return echo so that further return echoes arenot rejected or overpowered by a return echo at the upper end of themeasurement probe 226, in some applications, it may be desirable tocreate an anomaly in the vicinity of the upper spacer 320 that will inturn generate a return echo having a repeatable signature, when it isdesirous, for example, to calibrate a distance between the PCB-innerelectrode transition and a point along the height or axial extent of theupper spacer 320, such as the top and/or bottom of the upper spacer forcalibrating the probe 226 for example. Since the height of the spacer320 between the upper and lower surfaces is known, calculating a value“X” as shown in FIG. 7 for example, which includes the height orthickness of the spacer 320 as well as any resilient layer 323, whichalso has a known thickness, it is possible to adjust a clock timingassociated with a processor, by comparing a calculated distance with theknown physical distance between return echoes from the top of theresilient layer and/or upper spacer, and the bottom surface of the upperspacer. Since a known number of clock cycles can be determined betweenthe return echoes associated with the distance “X”, it is possible todetermine a clock calibration factor so that the measured distanceequals the known distance. It is also possible to subtract the distance“X” (FIG. 7) from a liquid height measurement within the measurementprobe 226, for example, in order to determine a distance between thelower surface of the upper spacer and the liquid surface.

As shown in FIGS. 5, 6, and 14-16, a second, or lower, spacer 340 islocated in the inner annular measurement space 298 of the elongatemeasurement probe 226 at a lower end thereof between the inner surface297 (FIG. 6) of the outer electrode 290 and the outer surface 299 of theinner electrode 292.

In accordance with one embodiment of the invention, the lower spacer 340is constructed of a conductive or semi-conductive material andconfigured to create a short across the inner and outer electrodes 290,292 to thereby produce a large anomaly and thus a return echo with alarge negative slope during operation to signify the end of themeasurable length of the probe 226.

In accordance with a further embodiment of the invention, the lowerspacer 340 is constructed of an insulating material and configured toisolate the inner and outer electrodes, to thereby produce a largeanomaly and thus a return echo with a large positive slope duringoperation to signify the end of the probe.

In accordance with yet a further embodiment of the invention, the lowerspacer 340 is constructed of a material similar to the upper spacer 320and configured to minimize or eliminate any anomalies and thus minimizeor eliminate the creation of any return echo that might signify the endof the measurement probe.

In accordance with another embodiment of the invention, the lower spacer340 is constructed of a semi-conductive material and configuration tocreate a small anomaly and thus a small return echo with a smallpositive or negative return echo to signify the end of the measurementprobe. With this last embodiment, measurement of liquid or materiallevel in close proximity to the lower end of the measurement probe canbe realized without interference from a larger end-of-probe return echo.

The lower spacer 340 includes a circular body 342 with an upper surface348, a lower surface 350, and a side surface 352 that extends betweenthe upper and lower surfaces. Openings 344 extend generally axiallythrough the body 342 between the upper surface 348 and lower surface350. The openings 344 surround a center depression 346 that is formed inthe upper surface 348 and has a lower depression surface 354 thatreceives the lower end 355 (FIGS. 5, 6) of the inner electrode 292. Theopenings 344 are preferably arranged so liquid or other material from acontainer 214 (FIG. 4) or the like can enter the inner annularmeasurement space 98 for monitoring liquid level and/or other propertiesof the liquid by the TDR measurement system 210. An annular groove 356is formed in the side surface 352 for receiving an O-ring (not shown) toseal the lower spacer 340 against the inner surface 297 of the outerelectrode 290. The lower spacer 340 can be installed in the outerelectrode 290 and over the inner electrode 292 through press-fitting,mechanical fastening, adhesive bonding, or other known connecting means.

Although one configuration of the lower spacer 340 has been shown, itwill be understood that the lower spacer 340 can have many differentconfigurations to accommodate a particular material to be measured, adesired return echo profile or elimination of a return echo at the lowerend of the measurement probe, as discussed above. It will be furtherunderstood that the lower spacer 340 can be eliminated, so long as theinner and outer electrodes are adequately supported, for example aspreviously described with the provision of one or more intermediatespacers 150 (FIG. 3).

Referring now to FIG. 7, a TDR measurement system 210A is configured asa linear measurement probe with the elimination of the lower spacer 340of the TDR measurement system 210, and the addition of a hollow shaft360 that surrounds the inner electrode 292 and fits within the innermeasurement space 298 of the elongate measurement probe 226A, which issimilar in construction to the elongate measurement probe 226. A plunger362 can be provided at the upper or inner end 363 of the shaft forcreating a large anomaly within the measurement space 298, which in turncreates a return echo large enough to be recognized as the position ofthe top surface 364 of the plunger 362. The shaft 360 and plunger 362can be integrally formed during manufacture and constructed of amaterial, such as a conductive or semi-conductive material that has adielectric different than the air or other fluid within the measurementspace 298. In this manner, the change in dielectric constants betweenthe air and the plunger that causes the creation of a sufficiently largereturn echo, which can be identified as a valid return echo. It will beunderstood that the plunger 362 can be eliminated, especially when theshaft itself causes a sufficiently large return echo.

In use, as the shaft moves in and out of the inner measurement space298, as represented by the double arrow 366 in FIG. 7 the location ofthe return echo will change accordingly. Such location can be monitoredin real time and used to control relative movement of two objects. Forexample, the outer electrode or the mounting head can be adapted formounting to one object, and the shaft can be adapted for mounting toanother object. When movement between the objects occurs, the Transducer210A can identify the movement to a very high degree of accuracy andrelay information related to the movement such as distance, speed, andacceleration, which can be used for machine control in automatedassembly lines and robotics, for example, or anywhere the measurement oflinear motion is desired. to be identified.

With particular reference to FIGS. 18A and 18B, the PCB 58 includesvarious electronic components 361, such as capacitors, resistors,inductors, one or more processors, amplifiers, diodes, transistors,comparators, and so on, as will be described in further detail below,arranged at various positions on the upper surface 111 and/or lowersurface 113 (FIGS. 3, 6 and 7) of the PCB 58. As shown in FIG. 18B, acalibration trace 365 is formed on one of the surfaces 111, 113, and/orintermediate surface(s) or layer(s) 119 of the PCB 58, it beingunderstood that the intermediate layer 119 can represent one or moreinternal layers and/or surfaces associated with one or more intermediatelayers between top and bottom layers in a multi-layer PCB. When formedon one or more internal surfaces or layers 119 (FIG. 3), the calibrationtrace can be isolated from other electronic traces, so that a dedicatedsurface or layer for the calibration trace 365 is provided. Inaccordance with one exemplary embodiment of the invention, thecalibration trace 365 extends across a substantial area of the PCBbetween a first end 369 and a second end 371 of the calibration trace,so that the calibration trace is approximately the same length of themeasurement probe 26. With the calibration trace being approximatelyequal in length to the measurement probe 26, the actual distancemeasurement can be determined without the cumulative errors associatedwith a much shorter calibration trace. By way of example, if theelongate measurement probe is approximately two feet in length, it ispreferred that the calibration trace also be approximately two feet inlength. If the measurement probe is one foot in length however, atwo-foot long calibration trace will increase the accuracy ofmeasurement even more. In this manner, high accuracy can be achievedwith the measurement of liquid level, granular material level, linearmovement or distance between the plunger 362 (FIG. 7) and themeasurement probe 226, the dielectric constant of the material beingmeasured, and so on.

With the size of the PCB being limited to fit within a housing orchamber of a particular size, the length of the calibration trace cangreatly vary, and need not be approximately equal to the length of themeasurement probe. By way of example, the calibration trace can rangebetween about 0.1 inch to over 100 inches or even much greater lengthsdepending on the dimensional constraints of the PCB, how manyintermediate or other layers the calibration trace is divided betweenand connected via conductive thru-holes to maximize the length of thecalibration trace, as well as the width of the calibration trace, andthe spacing between rows of the calibration trace. Likewise, the lengthof the measurement probe can range anywhere from 0.25 inch to over 100yards or even extend to much greater lengths. Accordingly, although inone exemplary embodiment it is preferred that the calibration tracelength and measurement probe length be approximately equal, it will beunderstood that the invention is not limited thereto, but the overalllength of the entire waveguide or transmission line, which includes boththe calibration trace and the elongate electrodes, can greatly varydepending on the measurement constraints of a particular installation orapplication of the TDR measurement system and the size limitations ofthe PCB as dictated by the configuration of the mounting head or otherhousing or structural limitations for receiving the PCB.

In order to facilitate description of the invention, the calibrationtrace 365 will be described as being associated with the intermediatelayer or surface 119 (FIG. 3), it being understood that theconfiguration of the calibration trace can greatly vary. The first end369 of the calibration trace 365 is connected to the electromagneticpulse generating circuitry of the electronics assembly 34 so that theelectromagnetic pulse is transferred onto the calibration trace 365 andtravels along its length toward the second end 371. The second end 371of the calibration trace 365 is connected to the third conductiveopening or thru-hole 108 so that the electromagnetic pulse travels alongthe length of the inner electrode 92, 292 and returns along the outerelectrode 90, 290. The calibration trace together with the inner andouter electrodes defines a waveguide or transmission line along whichthe electromagnetic pulse propagates during transmission thereof.

The physical length of the calibration trace is known and can be used inconjunction with the measured electronic length of the calibration traceto calibrate the clock cycle of the microcomputer 83 (FIG. 20) or otherprocessing means, and thus the electronic length of the calibrationtrace so that accurate distance to an anomaly (such as the change indielectric constants between media) within the elongate measurementprobe 26, 226 can be determined. One or more predefined anomalies can beinserted or created at one or more locations along the calibration traceto thereby create one or more calibration trace return echoes forcalibrating the transducer to a high degree of precision.

In accordance with one embodiment of the invention, the insertedanomalies or discontinuities can be mechanical in nature, such as achange in the width or thickness of the calibration trace, a transitionbetween the calibration trace and one or more of the elongate electrodes(and thus a discernible change in dielectric properties), and theinclusion of one or more spacers in the volume between the inner andouter electrodes at predetermined locations, which will change thedielectric constant whether immersed in air or liquid.

In accordance with a further embodiment of the invention, the insertedanomalies or discontinuities can comprise one or more electroniccomponents, such as transistors, biased diodes, switches, and the like,associated with the calibration trace and/or electrodes that can beselectively activated and deactivated either manually or automaticallythrough processor control, at intermediate and/or end locations alongthe calibration trace 365 and/or electrodes to thereby create one ormore identifiable return echoes that can be used for calibrating a clockor the like associated with the microcomputer.

No matter what embodiment is used for identifying one or more pointsalong the transmission line, including the calibration trace and/or theelectrodes, including a combination of mechanically- andelectrically-induced calibration anomalies, the one or more calibrationanomalies can be used for calibrating the clock or the like associatedwith the microcomputer, as well as timing circuitry associated withgenerating transmit and receive signals, as will be described in greaterdetail below, for electronically determining a start point, intermediatepoint, and/or end point of the calibration trace 365, as well as thedistance(s) therebetween. The start, intermediate, and/or end point(s)of the electrodes can also or alternatively be calibrated to correlatethe actual length of the calibration trace (or electrodes) with themeasured electrical length. In this manner, the physical length betweenthe known induced calibration anomalies, which can include a predefinedlength of the waveguide or transmission line, such as a portion of thecalibration trace or the entire length thereof, the combination of thecalibration trace and electrodes or portions thereof, and so on, iscorrelated with the electronically measured length between the inducedanomalies (the “electronic length”) as determined by the distancebetween the return calibration echoes, to ultimately attain highaccuracy and repeatability in measurement of the medium between theelongate electrodes.

In accordance with a preferred embodiment of the invention, the physicallength of the calibration trace is approximately equal to the physicallength of the elongate electrodes. In this manner, greater measurementaccuracy of the medium under consideration over prior art transducerscan be achieved. However, it will be understood that the length of thecalibration trace is not limited to the length of the measurement probeor electrodes, but can be of any reasonable length to obtain acceptablemeasurement accuracy in accordance with standards dictated by differentindustries. For example, measurement of liquid level within a fuel tankmay be held to a lower level of accuracy than measurement of linearmovement between critical components in machining operations.Accordingly, the length of the calibration trace and/or the distancebetween induced anomalies associated with the calibration trace can beselected to meet, exceed, or even greatly exceed industry standardswithout an increase in manufacturing costs.

Referring now to FIG. 19, a simplified schematic block diagram 370showing the basic relationship between components of the electronicsassembly 34 and the measurement probe 26 and 226 of the TDR measurementsystem 10 and 210, respectively, mounted in a tank or container 372 fordetermining the level 374 of liquid 376 within the container, as well asother conditions, in accordance with the invention is illustrated. Asshown, block 378 is representative of a microcomputer (U1 in FIG. 22)which includes the microcomputer 83 (FIG. 20). The microcomputer is inturn connected to analog circuitry 380 associated with the generation ofa transmit (TX) pulse delay (block 380) and the generation andtransmission of an electromagnetic pulse (block 382), along the lengthof the measurement probe 26, 226 for the purpose of determining materialheight or liquid level within the inner annular measurement space orvolume 98, 298. As will be described in greater detail below, thetransmit pulse occurs with picosecond resolution, which can be performedby the calibrated clock timing of low-cost processors, microcomputers,or the like. The processor is also connected to analog circuitry (block384) for generating an incremental receive (RX) delay signal uponreceipt of a RX generation signal from the microcontroller withnanosecond resolution. The Incremental RX Delay circuitry is in turnconnected to analog circuitry (block 386) for generating a samplereceive (RX) signal which in turn collects a sample reading or signalfrom the electromagnetic pulse traveling along the calibration trace 365associated with one of the layers of the PCB 58, such as intermediatelayer or surface 119 (FIGS. 7, 7A, 18B) and along the measurement probe26, 226 (FIGS. 1, 4). The nanosecond resolution is preferably generatedby the analog circuitry associated with block 384 to permit the use of alow-cost microcomputer. Once an analog measurement signal is received atblock 386, an analog to digital (A/D) converter (block 388) associatedwith the microcomputer converts the signal into digital form for furthersignal processing at block 390. Signals indicative of liquid level orother material level, linear movement, and so on, can then be stored inmemory associated with the microcomputer and sent to a display orfurther processing circuitry and/or routines for displaying and/oranalyzing the signal, as represented by block 392.

It will be understood that the term “microcomputer” as used herein isnot limited to a single system on a chip (SoC) device with one or morecentral processing unit(s) (CPU's), onboard memory (RAM, ROM, etc.),timers, ports, D/A converters, and so on, but can include a separateprocessor or processing unit that interfaces with analog and/or digitalcomponents required to execute one or more instructions of a softwareprogram for operating the TDR measurement system, including thegeneration of one or more analog and/or digital signals associated withelectromagnetic pulse transmission and/or reception at various times(and thus locations) along the length of the TDR measurement system fordetermining liquid level, material height, linear movement, and so on.

Accordingly, the present invention is not limited to a single type ofprocessing unit but can include any suitable processing means includingmicroprocessors, microcontrollers, microcomputers, processors,programmable logic chips (PLC's), ASIC devices, and/or processingsystems in digital and/or analog form so long as one or more of thevarious tasks associated with measuring the impedance at variouslocations along the calibration trace and/or along the measurement probeof the TDR measurement system and translating the resultant return echosignals into measurement values can be performed at least in part.Electronic components such as internal and external memory for storingprogram instructions and data, external and internal timers, D/Aconverters, and so on, can be provided as integral and/or separatecomponents and connected in a well-known manner for operation of TDRmeasurement system. Hence, it will be understood that the invention isnot limited to one type of processor or processing means for executingone or more instructions and/or timer and/or control functions, but mayinclude any equivalent structure and/or programming that changes thestructure of the processor, memory, and/or processor components toaccomplish, at least in part, one or more of the required tasks.

Referring now to FIGS. 20 and 22, a more detailed block diagram 400(FIG. 20) and schematic 402 (FIG. 22) of the electronics assembly 34 ofthe TDR measurement system in accordance with an exemplary embodiment ofthe invention is illustrated. In FIG. 20, the electronics assembly 34includes a plurality of different electronics modules that interfacewith the microcomputer 83 (U1) for operating the TDR measurement system.One of the modules includes the power supply 75 which, in accordancewith an exemplary embodiment of the invention, receives an approximaterange of power supply inputs between about 7 VDC and about 32 VDC, whichis typically provided by the vehicle, machine, system, or othermechanism associated with the TDR measurement system, and converts theinput supply voltage to 5 VDC operating output voltage to power thevarious electronic components of the electronics assembly 34. It will beunderstood that the power supply module 75 is not limited to theparticular supply ranges or the operating voltage as described, but maygreatly vary depending on the power available from the vehicle, machine,system, or other device, as well as the required operating voltage ofthe various electronic components associated with the electronicsassembly 34 of the invention.

Since the TDR measurement system may be used by vehicles or machineswith undesirable electrical noise, such as voltage spikes andvariations, transient voltages, EMI, back EMF, and so on, that couldrender inoperative one or more modules of the electronics assembly 34, apower regulator and filtering module 404 can be provided along with thepower supply 75 to ensure a stable supply voltage to the electronics andprotect the electronics from the undesirable electrical noise. Since theelectronics of the power regulator and filtering module 404 are knownand may greatly vary depending on the particular vehicle, machine orsystem associated with the TDR measurement system and the presence orabsence of undesirable electrical noise, the power regulator andfiltering module will not be further described. However, whereelectrical noise is filtered elsewhere, and/or a stable power supply isavailable, the module 404 or portions thereof can be eliminated.

An Equivalent Time Sampling (ETS) Delay Generator module 406 isconnected to the microcomputer 83 (U1) via a general interface module408. The interface module 408 can include bus strips (or the like) forproviding power to one or more of the modules, communication between themicrocomputer 83 and one or more of the modules, direct and/or indirectcommunication between modules, as well as passive and/or activecomponents for amplifying, filtering, buffering, converting signalsbetween analog and digital states, or otherwise modifying signalsassociated with the modules and the microcomputer.

The module 406 generates an incremental delay needed for equivalent timesampling (ETS) of an electromagnetic pulse transmitted many times duringa single measurement cycle. The module 406 includes provisions forhighly accurate timing associated with actuating the firing of manypulses during a measurement cycle that propagate along the waveguide tocreate an echo profile 405, as shown in FIG. 26 for example, and foractuating the receiver for sampling (and holding) data associated withthe echo profile created by each transmitted pulse.

One salient feature of the invention is the capability of initiallyreceiving data prior to actuating the transmission of electromagneticpulses so that the echo profile can be measured before the first pulseis transmitted and propagated along the waveguide, thereby ensuring thebeginning of an echo profile 405 (FIG. 26) or echo profile 407 (FIG. 27)for example, can be received and analyzed. As more and more pulses arefired in quick succession, the timing at which the data is receivedgradually changes from receiving data before pulse transmission toreceiving data after pulse transmission. In this manner, data associatedwith the end of the echo profile after the last pulse transmission canalso be received and analyzed. Accordingly, the entire echo profile 405or 407 for example, from before the first pulse transmission to afterthe last pulse transmission, representative of the impedance of the TDRmeasurement system along the entire length of the waveguide, can bereceived during a single measurement cycle for determination of liquidlevel and other measurable conditions. Preferably, several measurementcycles of plural transmissions are also performed and averaged orotherwise statistically combined for increased reliability of themeasurement data.

The ETS module 406 generates both a transmit timing signal forgenerating the electromagnetic pulse on the waveguide, and a receivetiming signal for actuating receipt of a single data point along theecho profile during a single transmission. Each subsequent transmittedpulse increases by a predetermined time interval or segment ΔT,designated as numeral 409 (FIGS. 26, 27) longer than the precedingpulse, to thereby generate and capture a data point representative ofthe impedance and change in impedance at a particular position where thedata point is captured along the waveguide, including the calibrationtrace 365 (FIG. 18B) and/or the elongate electrodes 90 and 92 (FIG. 3)or 290 and 292 (FIG. 6). This is most clearly shown in FIGS. 26 and 27for example, where the increasingly longer time intervals, representedby arrows of increasing lengths or multiples of the time interval ΔT at410A, 410B, 410C, . . . , 410Z in FIG. 26, and 410A, 410B, 410C, and soon, in FIG. 27. Arrow 410A represents the shortest time interval ΔT atthe beginning of a measurement cycle, while arrow 410W in FIG. 26 andarrow 410AH in FIG. 27, represents the longest time interval comprisingthe sum of multiple time intervals, where the last data point associatedwith the radar reflection along the waveguide is captured. FIGS. 26 and27 represent different time intervals or segments for ΔT, with FIG. 27having more transmit pulses than FIG. 26, so as to illustrate theflexibility of the invention.

In order to further illustrate the invention, FIG. 23 shows the outerand inner electrodes sliced into time segments or intervals 418 of ΔTduration, which represent distance segments based on the speed of theelectromagnetic pulse along the electrode, which is in turn dependent onthe dielectric value of the air, fluid, or solid material located in theannular inner measurement space 98, 298. Each subsequent transmissionincreases the transmission time by increasing multiples of ΔT, asrepresented by arrows 410, 412, 414 and 416, for example, to create theimaginary slices or segments 418 representative of distance traveledalong of the inner and outer electrodes for multiple transmissionsduring a measurement cycle where data points associated with thelocalized impedance associated with each segment or slice can begathered. The impedance value associated with each transmission isdependent on the localized dielectric constant of the air, fluid orsolid material located in the space 98, 298 between the electrodes atthe imaginary sliced locations or segments. The impedance values aregenerated during transmission and collected during reception todetermine the level of liquid or other measurable properties of themedia within the space 98, 298.

As shown, the energy of the electromagnetic pulse travels from the innerelectrode 92, 292 to the outer electrode 90, 290 via the annular innermeasurement space or volume 98, 298, as represented by radiallyextending arrows 415. The value of the impedance at any location orperiod of time along the length of the measurement probe can beexpressed by the following formula:

$\begin{matrix}{\frac{C}{L} = \frac{2\pi \; k\; \epsilon_{0}}{\ln \left( \frac{b}{a} \right)}} & (1)\end{matrix}$

Where: C is capacitance; L is unit length; k is the dielectric constant;∈_(o) is the dielectric permeability of free space (air in the spacebetween the conductors=1); a is the inner radius of the outer electrode;and b is the outside radius of the inner electrode. When the elongatemeasurement probe is arranged generally vertically in a tank, and whenliquid is located in the annular inner measurement space between theelectrodes, part of the elongate measurement probe will be filled withliquid and cause a change in impedance beginning at the air/liquidinterface. The change in impedance creates a return echo, where some ofthe energy of the electromagnetic pulse is reflected back to theelectronics where it can be analyzed and determined whether the level ofliquid has indeed been located, through known analysis techniques byexamining the properties of the return echo, such as amplitude, area,and whether a return echo is expected at the determined distance alongthe waveguide comprising the calibration trace and the elongateelectrodes.

Accordingly, each subsequent transmission during a measurement cyclecaptures a data point at a different location. For example, if thelength of the calibration trace is 500 mm and the length of the elongatemeasurement probe is 500 mm, and 1,000 transmissions of electromagneticenergy pulses or bursts are activated during a measurement cycle, thedistance between data points will be approximately 1,000 mm/1,000transmissions=1 mm distance between data points. The segments 418 inFIG. 3 then represent 1 mm distance between measurements. Of course, theamount of transmissions, as well as the lengths of the calibration traceand electrodes can greatly vary. If for example 2,000 transmissions overthe 1,000-mm total length occurs, the measurement resolution, ordistance between transmissions, will be 0.5 mm. If 100,000 transmissionsoccur over the same length for example, 100,000 data points will begathered with a resolution of 0.01 mm distance therebetween.

Furthermore, when the bursts of electromagnetic energy occur at afrequency of 2.4 Ghz for example, which is within the capabilities ofvery low-cost microcomputers having an internal clock, the microcomputer83 is used in conjunction with the analog components of the module 406,to create first time intervals between actuating the receiver (RX) forreceiving adjacent data points in the nanosecond range, and second timeintervals in the picosecond range beyond the nanosecond range of eachsubsequent RX time interval for actuating the transmitter (TX) fortransmission of the electromagnetic energy bursts. Actuating thetransmitter picoseconds after receiver actuation allows data collectioneven before the first electromagnetic energy pulse occurs, as describedabove, to thereby collect data before, during, and after transmission ofthe electromagnetic pulse, as shown for example in FIGS. 26 and 27, aswill be described in further detail below.

Thus, resolution of the TDR measurement system 10, 210 can greatly varydepending on the number of transmissions that will be actuated over theelectronic length of the TDR measurement system. Other units of measurecan be used without departing from the spirit and scope of theinvention. More details of module 406 will be described below withreference to FIG. 22D.

A first calibration module 420 for generating a first calibration markin the form of a first calibration return echo at a first location alongthe calibration trace 365 can be provided. Likewise, a secondcalibration module 422 can be provided for generating a secondcalibration mark in the form of a second calibration return echo at asecond location along the calibration trace 365. Preferably, the firstcalibration mark is at an intermediate location along the length of thecalibration trace 365, while the second calibration mark is at the endof the calibration trace 365 to mark the end of the calibration traceand the transition between the calibration trace and the elongatemeasurement probe 26, 226.

The first and second calibration modules 420 and 422, respectively,provide selectable first and second discontinuities, respectively, ofpredefined proportions to thereby selectively generate respective firstand second calibration return echoes 424 and 426 (FIGS. 26 and 27) forexample, during a calibration cycle, which can occur during eachtransmission, during each measurement cycle comprising a plurality oftransmissions, or whenever it has been determined that a sufficientchange in ambient temperature has occurred to affect the dielectricconstant of the material to be measured, the clock timing from themicrocomputer, and so on.

The first and second calibration echoes 424, 426 can be analyzed todetermine the electronic distance therebetween, i.e. the electronicallymeasured distance between the first and second calibration echoes, theslope between the echoes, the size and shape of the calibration echoes,the area under the calibration echoes, and so on, in order to correctfor less accurate or inconsistent clock timing pulses associated withvery low-cost microcomputers. Since the physical distance between thediscontinuities is known, and the electronic distance can be measured,any discrepancy between the two values can be resolved to obtain highlyaccurate clock timing cycles that would exceed the accuracy of the clockpulses of much more expensive microcomputers. In this manner, the costof the TDR measurement system can be significantly lowered by specifyingin most cases very low-cost components for the electronics assembly 34.

It will be understood that one or more of the first and secondcalibration modules can be eliminated, especially when the distancebetween the start of the calibration trace 365 to the first or secondselectable discontinuity is physically known and can be electronicallymeasured to thereby correlate any discrepancies.

In accordance with a preferred embodiment of the invention, since thesecond calibration module 422 is located at the end of the calibrationtrace 365, it is advantageous to keep the second calibration module andeliminate the first calibration module 420 since the longer distancebetween the beginning of the calibration trace and the ending thereofwould be expected to yield greater accuracy than the shorter distancebetween the first and second calibration modules. It will be furtherunderstood that more than two calibration modules can be provided whenit is desirous to obtain a greater number of calibration points alongthe calibration trace 365.

A RF transmit pulse generator 428 is electrically connected to the ETSdelay generator module 406 for imposing an electromagnetic energy pulse,preferably comprising a radio frequency (RF) energy pulse, such a radarenergy pulse, on the waveguide including the calibration trace 365 andthe measurement probe in accordance with the timing intervalsestablished by the ETS delay generator module 406 and the microcomputer83, as discussed above with respect to FIG. 23.

A RF receive pulse generator module 430 (FIG. 22) is electricallyconnected to the ETS delay generator module 406 for generating the RFreceive pulse to collect data related to the RF energy pulse imposed onthe waveguide comprising the calibration trace 365 and the measurementprobe, including return echoes due to anomalies or discontinuities,changes in dielectric constant, and electrical shorts between theelectrodes as discussed above, to signify the end of the calibrationtrace and/or measurement probe, for example, in accordance with thetiming intervals established by the ETS delay generator module 406 andthe microcomputer 83.

A RF receiver module 432 (FIG. 22) includes a RF bias generator 434(FIG. 22) that is operably connected to a receive switch module 436 forbiasing the module 434. The RF bias generator functions as a DC servo tomaintain a constant bias on the receive switch module 436, resulting inconstant sensitivity of the module 436 to the sample pulses and thereceived data generated by the imposed RF energy pulse.

The receive switch module 436 is operably connected to the RF receivepulse generator module 430 and controls when data is received inaccordance with the timing intervals established by the ETS delaygenerator module 406 and the microcomputer 83.

The RF receiver module 432 is operatively associated with the RF receivepulse module 430 to generate a second sample pulse from the primarysample pulse associated with the RF receive pulse module 430. The secondsample pulse allows the system to use a second track and hold amplifiermodule 437 which greatly amplifies the received signal upon actuation ofa sample pulse generator module 438. The module 438 is operablyassociated with the receive pulse module 430 and the receive switchmodule 436 for greatly increasing the received measurement data signalfrom the receive switch module 436.

Details of the RF receiver module 432, including the RF bias generator434, second track and hold amplifier module 437, and the sample pulsegenerator module 438, will not be described as they can be constructedof known analog components arranged in a circuit or the like forexecuting their respective functions. Such modules or componentspreferably work in conjunction with the analog circuitry associated withother modules of the electronics assembly 34, including themicrocomputer 83 that interface with electronic components of the othermodules or portions thereof for initiating and carrying out thefunctions of the RF receiver module 432 and its associated RF biasgenerator 434, track and hold amplifier module 437, and sample pulsegenerator module 438.

In accordance with a further embodiment of the invention, the receivermodule 432, including at least a portion of one or more of thecomponents or modules 434, 437, and 438, can comprise digital devices orcomponents and arranged in a known manner to accomplish their respectivefunctions. Such devices or components can also work in conjunction withthe microcomputer 83 and/or with other circuitry for accomplishing theirrespective functions.

In accordance with yet a further embodiment of the invention, at least aportion of the receiver module 432, including the components or modules434, 437, and 438, can be at least partially embodied as operatinginstructions associated with the microprocessor. Such instructionsenable activation and deactivation of predefined ports associated withthe microprocessor 83 for interfacing with the analog circuitryassociated with other modules of the electronics assembly 34 and thusexecuting the equivalent functions of the RF receiver module 432 and itsassociated RF bias generator 434, track and hold amplifier module 437,and sample pulse generator module 438.

The microcomputer 83 can also be programmed with dedicated ports togenerate one or more of the sample pulses, activate the second track andhold amplifier module 437, and/or programmed as software modules withinthe memory (not shown) of the microprocessor 83 for accomplishingsimilar purposes or functions.

A buffer amplifier module 440 is also operatively associated with thesample and hold module amplifier 437 and includes a high impedance inputbuffer amplifier for amplifying the received signal stored on acapacitor C35 of the module 440.

An analog low pass filter module 442 is operably connected to the A/Dconverter of the microcomputer 83, where the received signals aredigitized for further processing in the microcomputer.

A temperature sensor module 444 is operatively associated with themicrocomputer 83 for providing temperature compensation due to ambienttemperature fluctuations to the system, which not only affects themechanical dimensions of the compensation trace and the elongateelectrodes of the measurement probe, but also the dielectric constant ofthe materials of the system as well as the medium or material(s) to bemeasured.

A D/A converter module 446 is operatively associated with themicrocomputer 83 for converting a digital control signal output from themicrocontroller to an analog control signal that can be used foroperating one or more of the analog modules. The D/A converter module446 can also be used for generating an analog signal from digitalinformation stored in memory indicative of media or material condition,to thereby permit use of the TDR measurement system with analogindicator means, including visual and audio devices such as one or moreindicator lights, gauges, buzzers, and so on. It will be understood thatthe module 446 can be eliminated when only digital signals and/ordigital indicator means, such as digital displays or the like, will beused.

A voltage reference module 448 is operatively associated with the D/Aconverter module 446 for creating precision analog signals from thedigital signal output of the microprocessor that can be used foroperating one or more of the analog modules and/or generating an analogsignal indicative of material condition, such as liquid level when theRF transducer is embodied as a liquid level measurement transducer, orlinear travel between two objects connected to the transducer whenembodied as a linear transducer. Other material conditions can also becommunicated in analog form, as discussed above, for permitting a user,system, and so on, to receive, view, and/or interpret the informationrelated to the material condition and perform further steps if needed.

Whether the output signals reflective of the material condition, such asliquid level or linear movement, be analog or digital, a hard-wireddisplay 77 and/or a remote device linked wirelessly with the RFTransceiver 79, as previously described, can be used to relate theinformation indicative of liquid level, position, or other condition ofthe material between the elongate electrodes to a remote system ordevice.

Referring to FIG. 21, a block diagram of an independent communicationdevice 450 for receiving and displaying measurement data from the TDRmeasurement system is shown. The device 450 includes a microcomputer 452for processing signals received from a RF transceiver 454 operativelyassociated with the microcomputer 452 and in wireless communication withthe RF transceiver 79 of the TDR measurement system, for receivingwireless signals indicative of the material level, condition, linearposition, and so on, from the TDR measurement system. A display 456 isalso operatively associated with the microcomputer 452 for displayingthe material condition, and can display other information related orunrelated to the material condition. A user interface (UI) 458 can alsobe provided on the remote device 450 for changing display parameters,selecting data signals from other TDR measurement systems or otherwireless devices, operating application specific programs associatedwith the TDR measurement system, and so on. A power supply 460 is alsoprovided, such as a portable battery or the like, for powering theelectronic components of the device 450. It will be understood that theRF transceiver 79 and the device 450 can be eliminated when the TDRmeasurement system is used exclusively in systems, machines, or the likewhere information is directly communicated through a hard-wiredconnection, as previously described.

Referring now to FIGS. 22 and 22A-22J, a more detailed description ofthe various electronic components of the modules of the electronicsassembly 34, and their relationships, in accordance with an exemplaryembodiment of the invention, will now be undertaken. FIGS. 22A-22J, inparticular, are enlarged circuit diagrams of the different inventivemodules of FIG. 22 to facilitate the more detailed description of theanalog circuit elements of each module.

FIG. 22A is an enlarged schematic view of the microcomputer 83 and isdesignated as U1. As afore-mentioned, the microcomputer 83 is preferablyvery low in cost and therefore has limited computing capacity, clockfunction, a limited number of I/O ports for executing various functionsin accordance with the invention in order to measure one or moreproperties of the medium being measured, including liquid level, linearmovement, dielectric constant, and so forth. The functions arepreferably embodied as computer-readable instructions of a softwareprogram or program segments stored in memory or the like and accessibleby the CPU of the microcomputer 83 for executing the instructions andassociated functions including, but not limited to, clock timing,calibration of the clock timing, calibration of the TDR measurementsystem, actuation and operation of electronic components and theirrelated circuitry associated with other modules, data recording andretrieval, conversion between digital and analog signals and vice-versa,and so forth.

As shown in FIG. 22A, the microcomputer 83 includes power and groundports labeled VDD and VSS respectively, for providing electrical powerto the microcomputer. The microcomputer also includes an A/D converterinput associated with port P2.11, a clock signal output at port P0.15,and various other ports that will be described in conjunction with theother modules. A power supply filter circuit comprising capacitors C2and C11 in parallel between power and ground can also be included, itbeing understood that the filter circuit can greatly vary, as is wellknown, and can be eliminated when a stable power supply is available.

Referring now to FIGS. 22B, and 24-27, the ETS module 406 generates anincremental time delay for equivalent time sampling (ETS) of anelectromagnetic pulse transmitted several times during a singlemeasurement cycle. Each subsequent transmitted pulse increases by apredetermined time interval ΔT beyond the previously transmitted pulseso that each transmitted pulse is ΔT longer than the preceding pulse. Asaforementioned, the term “pulse” as used herein refers to adistinguishable burst, ramp, wave, or other rapid change inelectromagnetic energy, such as a change in amplitude or frequency of asignal imposed on the waveguide or transmission line of the TDRmeasurement system, e.g. that portion of the pulse that remains highduring transmission, as shown in FIGS. 26 and 27 for example. Eachmeasurement cycle has a duration T at least as long as the time requiredfor the electromagnetic pulse to travel at least the entire length ofthe waveguide, including the distance between the beginning of thecalibration trace 365 (FIG. 18B) and the end of the elongate electrodes90 and 92 or 290 and 292, of the measurement probe 26 or 226 (FIGS. 2and 5).

As described above, the calibration trace and electrodes togethercomprise a waveguide or transmission line with a total combined lengthfor guiding the electromagnetic pulse therealong from the beginning ofthe waveguide to the end thereof. The electromagnetic pulse preferablycomprises a portion of a square wave pulse or the like in the radarfrequency range of the electromagnetic radiation spectrum. The radarwave typically travels at the speed of light when unimpeded, e.g. in aperfect vacuum, but due to differences in the dielectric constant of airand various materials, the radar wave can actually slow down to half thespeed of light or less, depending on the dielectric constant of thematerial or fluid through which the radar wave propagates. Accordingly,although the waveguide length is relatively long, the duration of theradar wave is very short and can thus be transmitted thousands of timesper second, for example, during a single measurement cycle. Preferably,several measurement cycles with thousands of transmissions of the radarwave are performed to obtain data that can be analyzed for determiningliquid level or other measurable characteristics of the medium as wellas the interface between media.

As shown in FIGS. 23 and 26, each transmission pulse is associated withreaching a particular distance or location along the waveguide of theTDR measurement system to thereby generate and capture a single datapoint per transmitted pulse at a particular position along thewaveguide, such as the calibration trace 365 or elongate measurementprobe 26, 226 representative of the electrical state, i.e. the impedanceor change in impedance, at the particular position where the data pointis captured along the waveguide. In order to generate the signals forinitiating the transmit and receive signals, the microcomputer 83 (U1)generates two timing signals, namely SignalSlow associated with outputport P0.6 and SignalFast associated with port P0.9 of the microcomputer.SignalSlow for example, can comprise a 40 Hz square wave for controllingthe start of a measurement cycle. SignalFast, for example, can comprisea 4 MHz square wave for controlling the start of each radar or radiofrequency (RF) pulse. The RF pulse occurs several times during themeasurement cycle for the purpose of collecting a single data point onetransmission at a time along the waveguide each time the RF pulse isgenerated and propagated along the length of the waveguide. Both signalsfrom the microprocessor, in accordance with corrected or calibratedclock timing, are input to the ETS delay generator module 406 whereupontwo signals are output, namely, SignalTransmit and SignalSample.SignalTransmit controls the timing of the RF transmit pulse ortransmitter, while SignalSample controls the timing of the RF sampler orreceiver. It will be understood that the SignalSlow and SignalFastvalues are given by way of example and can greatly vary withoutdeparting from the spirit and scope of the invention.

SignalTransmit is applied to the sample pulse generator module 438 andtriggers transistor Q3 (FIG. 22C) in the RF transmit pulse generator428, which generates a fast falling edge of a pulse at the drain 468 oftransistor Q3. For example, the fast falling edge can be generated for a130 picosecond or 2.7 GHz bandwidth, depending on the values of thepassive electronic components. This pulse couples through capacitor C42(FIG. 22F) of the receive switch module 436 to the waveguide connection462, which is electrically connected to the first end 369 (FIG. 18B) ofthe calibration trace 365 for propagating the RF signal down thewaveguide. The fast falling edge propagates down the waveguide and isreflected in part by the surface of the liquid or material in the space98, 298 (FIGS. 3, 6) between the electrodes to create an echo signal.The echo signals travel back up the waveguide and are coupled throughthe capacitor C42 (FIG. 22F) of the receive switch module 436 to theanode of diode D1. It will be understood that the timing of the fastfalling edge is given by way of example only and can greatly varywithout departing from the spirit and scope of the invention.

The module 406 (FIG. 22B) SignalSample connects to the RF receive pulsemodule 430 (FIG. 22D) on the SMP line 464 to turn on the transistor Q4and generate a fast edge at the drain 466 of the transistor Q4. Forexample, the fast edge can be generated for a 130 picosecond or 2.7 GHzbandwidth, depending on the values of the passive electronic components,as will be described below. The drain 466 of transistor Q4 is coupled tothe cathode 468 (FIG. 22F) of diode D1 through a pulse forming networkincluding resistor R18 in module 430 (FIG. 22D), the calibration trace365 (FIG. 18B), and the capacitors C27 and C26 in module 436 (FIG. 22F).The resulting pulse at the cathode 468 of diode D1 forward biases thediode, which allows the received echo signal to couple through theresistor R20 in module 436 to the RF bias generator 434 (FIG. 22) thatis operably connected to the receive switch module 436 for biasing thediode D1 of the module 434. It will be understood that the values of thepassive components can vary for varying the speed of the edge at thedrain 466 of transistor Q4, without departing from the spirit and scopeof the invention.

The resistor Rd of the module 436 (FIG. 22) is in parallel with thecapacitor C42 and has a relatively large value, such as in the kilo-Ohm(KΩ) range for example, to allow the current to safely drain from thecapacitor C42 of the module 436 (FIG. 22F) when the forward bias on thediode D1 applied by the RF bias generator 434 (FIG. 22) is removed. Withthis feature, electric sparks that might otherwise occur due to theremoval of the forward bias on the diode D1 are safely eliminated. Thetransducer is therefore capable of measuring the level of liquid thatmay be flammable, explosive, or otherwise react negatively to anelectrical spark.

With particular reference to FIGS. 22A, 22B, and 24, the Equivalent TimeSampling (ETS) Delay Generator module 406 is and its operation forgenerating receive and transmit pulses to the receive module 430 (FIG.22) and transmit module 428 (FIG. 22), respectively, will now bedescribed in greater detail. The TDR measurement system uses ETS wheremany individual RF transmit/receive cycles are initiated during a singlemeasurement cycle. During each transmit/receive cycle, a single datapoint of the complete echo profile is captured. In each successive RFtransmit/receive cycle, the delay between transmit and receive isincreased, so that with a plurality of cycles at predetermined timedelay intervals, a complete echo profile can be captured and analyzed.See, for example, echo profile 405 in FIG. 26 or echo profile 407 inFIG. 27. One of the primary purposes of ETS module 406 is to generatethe incremental delay between transmit and receive pulses in a highlyefficient, accurate, and low-cost manner without the need for arelatively large number of precision electronic components and theirattendant higher costs, as required by prior art solutions.

The ETS Module 406 includes a dual inverter, labeled U3, with powerinput at pin 5, ground at pin 2, open drain outputs at pins 4 and 6, andsignal inputs at pins 1 and 3. Pin 1 receives a slow pulse signal frompin 9 of the processor (microcomputer 83) Likewise, pin 3 receives afast pulse signal from pin 12 of the processor U1. A measurement cyclebegins when SignalSlow goes low in response to a slow signal from theprocessor U1, which opens the drain output (pin 6 of U3) and allows thecapacitor C9 to begin charging through the resistor R10, therebycreating a RC charging circuit with the step 1 470 (FIG. 22B). Thevalues of the capacitor C9 and resistor R10 are selected so that arelatively slow charge in voltage across the capacitor C9 occurs.

As shown in FIG. 24, the slow charge rate can be expressed as a functionof voltage versus time with the values of the capacitor and resistorselected so that a slow curve 470 is generated as voltage versus timeas:

$\begin{matrix}{V_{1} = {V_{cc} \times \left( {1 - e^{\frac{- t}{t_{s}}}} \right)}} & (2)\end{matrix}$

Where V₁ is the charging voltage across the capacitor C9, V_(cc) is thesupply voltage, t is the elapsed time since application of the supplyvoltage, and t_(s) is the RC time constant of the RC charging circuit.

The frequency of SignalSlow is selected for example at 40 Hz, asprogrammed into or otherwise set by the processor U1, so a completemeasurement cycle is repeated every 25 milliseconds. It will beunderstood that other values can be chosen for the frequency ofSignalSlow without departing from the spirit and scope of the invention.

The SignalFast is applied to pin 4 of the second inverter U3 andcontrols the beginning of the RF transmit/receive cycle. The frequencyof SignalFast is selected at 4 MHz, for example, by the processor U1 asprogrammed or otherwise set, with the resultant period being 250nanoseconds. Accordingly, there are 25 mm/250 nm=100k RFtransmit/receive pulses per complete measurement cycle with theexemplary values. It will be understood that other values can be chosenfor the frequency of SignalFast without departing from the spirit andscope of the invention.

As shown in FIG. 24, the fast charge rate can be expressed as a functionof voltage versus time with the values of the capacitor and resistorselected so that a fast curve 472 is generated as voltage versus timeas:

$\begin{matrix}{V_{2} = {V_{cc} \times \left( {1 - e^{\frac{- {({t - t_{0}})}}{t_{f}}}} \right)}} & (3)\end{matrix}$

Where V₂ is the charging voltage across the capacitor C9, V_(cc) is thesupply voltage, tt is the elapsed time since application of the supplyvoltage, t₀ is the time at which the transmit signal is generated, andt_(s) is the RC time constant of the RC charging circuit.

Referring to FIGS. 22B and 24, the signal slow curve is initiated attime t=0, while the signal fast curve is initiated at to. Initiation ofthe fast curve is also when the transmit signal is actuated. Uponintersection of the slow and fast curves, as shown at time ti in FIG.24, the receive signal is actuated. In accordance with equations (2) and(3) above representing the slow and fast curves, respectively, when thefollowing occurs:

$\begin{matrix}{{V_{2} = {V_{1}\text{:}}}{{then},}} & (4) \\{{{V_{cc} \times \left( {1 - e^{\frac{- t}{t_{s}}}} \right)} = {V_{cc} \times \left( {1 - e^{\frac{- {({t - t_{0}})}}{t_{f}}}} \right)}}{or}} & (5) \\{\left( {1 - e^{\frac{- t}{t_{s}}}} \right) = \left( {1 - e^{\frac{- {({t - t_{0}})}}{t_{f}}}} \right)} & (6)\end{matrix}$

In this instance, Vcc cancels out, and a reading is taken (data pointreceived) at the intersection of the two non-linear equations at timeti. Over time, as equation (2) continues to rise during the measurementcycle, it will take longer and longer for equation (3) to intersectequation (2) as defined by the analog components of the RC circuitspreviously described. This results in a longer delay, preferably inmultiples of picoseconds, for receiving the return pulse and thus afurther position along the waveguide where the impedance is measured,and the data point associated with the localized impedance is received.This process takes place in the dual comparator, represented bydesignators U2:A and U2:B of the ETS module 406 in FIG. 22B. As shown,comparator U2:B sends a transmit signal on line TR denoted by numeral474 associated with both the output of comparator U2:B of module 406 andthe RF transmit pulse generator 428 (FIG. 22C). In accordance with anexemplary embodiment of the invention, the timing of the transmit signalis determined by the microcomputer 83, and occurs for example at every250 nanoseconds. Likewise, the intersection of the fast and slow curvesoccurs every 250-nanosecond cycle, plus multiples picoseconds to cause areceive signal to be generated a predefined distance each transmissioncycle to thereby eventually capture the entire echo profile. Thus, theintersection of the fast and slow curves causes the comparator U2:A totrip as the value of capacitor C7 equals the value of capacitor C17,both of which are inputs on pins 1 and 2 of the comparator U2:A.Preferably, one transmit pulse occurs at 250 ns, while the correspondingreceive pulse occurs at 250 ns plus 1 picosecond. Likewise, the nexttransmit pulse occurs at 400 ns, while the corresponding receive pulseoccurs at 500 ns plus 2 picoseconds. Again, the next transmit pulseoccurs at 750 ns, while the corresponding receive pulse occurs at 750 nsplus 3 picoseconds, and so on, for thousands of transmit cycles untilthe entire echo profile is captured and stored, where it cansubsequently be analyzed.

Over time, although the capacitors C7 and C17 (and thus the equations(1) and (2)) are nonlinear, due to the increasing delayed timing of theintersection of the fast and slow curves, the remaining values, once Vccis canceled out, as discussed above, reveals a linear function asfollows:

$\begin{matrix}{t = \frac{- t_{0 \times t_{s}}}{t_{f} - t_{S}}} & (7)\end{matrix}$

See FIG. 25 for example which shows three sets of data points for thetransmit signal at 250 ns, 500 ns, and 750 ns, and three sets of datapoints for the receive signal intersecting with their respectivetransmit signals, due to the very fast rise of the fast curve and thevery slow rise of the slow curve, only a single receive data point canbe represented at this scale for each transmit signal. However, it isclear from this chart that the intersection of the fast and slow curvesat the predetermined transmit intervals results in a linear curve whenconnecting the intersection of the fast and slow curves. The linearnature of multiple fast and slow curve intersections is independent ofthe timing of the transmit signals. It will be understood that althoughcertain values have been given for the transmit and receive signalactuation and intersection, a wide variety of values can be selecteddepending on a particular application, how much accuracy or resolutionis desired, the processing speed of the microcomputer, and so on.Accordingly, the present invention is not to be limited by the exemplaryvalues set forth herein.

The purpose of using the ETS method in accordance with the invention isdue to the high-speed propagation of the radar wave along the waveguide,e.g. the calibration trace and elongate electrodes, as previouslydescribed. The radar wave moves at the speed of light in a vacuum, andclose to the speed of light in air or atmosphere. It has been foundthat, when a material is reached by the radar wave with a higherimpedance value than air, the propagation is much less than the speed oflight. Along the calibration trace for example, as buried in the PCB,the propagation is approximately half the speed of light. Accordingly,the entire echo signal for a 24-inch measurement probe occurs inapproximately four nanoseconds (4 billionths of a second). While it maytheoretically be possible to create electronics that can record the echothis fast, such an attempt would be very expensive and thus notpractical for most liquid level measuring transducers in transportation,marine, off-road, and other markets.

On the one-hand, by limiting the electronics assembly, includingsoftware instructions associated with the microprocessor, to onlycapturing the echo from one distance per cycle, the transducer can be ofsufficient low-cost to be competitive in the above-mentioned markets.Until the present invention, this has not been achieved.

On the other hand, for a 24-inch probe, thousands of measurement cyclesmay be required to capture the entire echo profile, such as the echoprofile 406 in FIG. 26, for example. However, this compromise is nottypically an issue, since each individual measurement only takes 250 ns.The TDR measurement system as presently embodied is capable of easilycapturing 40 complete echo profiles per second (as opposed to 25 millionecho profiles we could record at full speed). Since the level of liquidin a tank or the movement of a plunger within the outer electrode doesnot significantly change in 25 ms, (much less 4 ns), even with the timetradeoff, the TDR measurement system is still much faster than ispractically needed, even with a microcomputer currently priced at muchless than $1 USD, and low-cost analog electronic components that do not,in and of themselves, offer consistent accuracy or reliability, yet suchcomponents, when properly combined and managed, as in the presentinvention, produces a highly accurate, highly repetitive, and highlyreliable transducer, with perhaps only a single comparator being of thehigh-precision variety. Another factor, which may be important dependingon the level of accuracy and repeatability desired, is the selection ofC7 and C9, which should have low or zero temperature and vibrationsensitivity. The comparator U2 should also have high input impedance andprovide fast-rising edges on outputs.

As shown in FIG. 22D, the fast sampling signal from transistor Q4 of theRF receive pulse generator module 430 couples with the sample pulsegenerator module 438, which delays the sample signal and delivers thedelayed sample signal to the sample input of the sample and holdamplifier module 437. When sampled, the output of the sample and holdamplifier module 437 charges the capacitor C35 associate with the bufferamplifier module 438 (FIG. 22G), which integrates the received echoreturn signals from multiple 4 MHz cycles.

Modules 437 and 438 together derive a second sample pulse (generated inmodule 438) from the primary sample pulse from the transistor Q4 in thereceive module 430. This second pulse allows the system to use a secondtrack and hold amplifier (module 437), which greatly increases thesignal from the sample diode D1 in module 436. The RF receiver module434 forms a DC servo to maintain a constant bias on D1, resulting inconstant sensitivity of D1 to the sample pulses and the received echoes.

As shown in FIG. 22H, the module 440 includes a high impedance inputbuffer amplifier for the integrated echo received data signal stored oncapacitor C35 of the buffer amplifier module 438. The module 440 alsoforms an analog low pass filter whose output is connected to the analogto digital converter (at pin 4) of the processor, where the echo signalis digitized and received in such a manner into the microcomputer forstorage and analysis of the return echo profile.

As shown in FIG. 22I, a temperature sensor in module 444 provides fortemperature compensation. Module 446 is associated with module 444 andincludes a digital to analog converter and a voltage reference forprecision analog indication of the liquid level, linear position, aswell as other material properties or positions of anomalies along thewaveguide.

As shown in FIG. 22J, the power supply module 75 includes a switchingpower supply that converts input voltages in the range of about 7 to 32VDC into 5 VDC in order to power the system. Although particular valuesof the electronic components, such as the value of capacitors,inductors, and resistors, and so on, have not been given, it will beunderstood that the values can greatly vary and can be selected to varythe RC fast and slow timing curves, as well as other functionsassociated with each module of the electronics assembly.

Referring now to FIG. 28, once the data has been gathered for at leastone measurement cycle, the microcomputer performs an analysis of thedata by creating a processor-generated time varying threshold (TVT)curve 480 that follows the curve of the originally gathered dataassociated with the measured return echo profile 405. The TVT curve canbe expressed for each data point as the average of a certain number ofreceived data values before and after the TVT data point on the returnecho profile, with a limit on the amount of slope the TVT curve isallowed to follow the return echo profile, such as represented bynumeral 486 in FIG. 28.

In this manner, unimportant anomalies or return echoes can be ignored,while important anomalies, such as calibration marks to calibrate thesystem clock and thus the measured distance to the anomalies, as well asthe top of a liquid surface within the inner space between theelectrodes to determine liquid level for example, can be automaticallylocated, such as represented by numeral 486, where the maximum slope ofthe TVT curve does not follow the slope of the echo profile 405.Likewise, important anomalies, such as the echo 482 created at thetransition of the electrodes and the calibration trace, to calibrate thetiming of the system clock, can also be identified. The commonality torecognizing a valid return echo is in the fast fall (or rise) of thereturn echo profile and the slope limitation of the TVT curve thatprohibits the TVT curve from following the slope of the echo profile405. The following expression can be used for generating the TVT curvein FIG. 28, it being understood that other expressions can be usedwithout departing from the spirit and scope of the invention:

$\begin{matrix}{{{TVT}(i)} = {{\sum_{i - m}^{i + n}\; \frac{\left( {i - m} \right) + \left( {i + n} \right)}{m + n}} - k}} & (8)\end{matrix}$

where i is given range count; and m, n and k are constants, withsuitable values including: m=20, n=10, and k=200. It will be understoodthat the values are given by way of example and can greatly varydepending on the nature of the TVT curve, the return echo profile, andpreferred behavior of the TVT curve.

Calibration of the system clock, for example the clock associated withthe low-cost microcomputer 83, can take place several ways. Oneexemplary embodiment of the invention comprises activating thetransistor Q1 in module . . . is calibration of the system clock. Whenturned on, Q1 provides a large and precise echo from the end of thecalibration trace 365 (delay line) buried in the PCB. Since the delayline is a calibrated distance, by measuring the length of the delay linewith the system, the system can be calibrated.

Referring now to FIG. 29, a block diagram illustrating a method 500 forcalibrating the calibration trace 365 (FIG. 18B) is disclosed. Thismethod follows the procedure as discussed above with determining theecho profile at least of the calibration trace, then comparing thedetermined length with the actual physical length taking into accountdimensional variables due to temperature fluctuations, then deriving andstoring a calibration factor, and using that calibration factor todetermine a system clock distance calibration. For example, if the endof the calibration trace 365 is reached in 1,000 clock cycles, and theexpected or actual number of clock cycles for the physical length is1,005, the calibration factor would be a ratio of the clock cycles toensure that the determined electronic length is equal to the actualmechanical length. Once the system clock has been calibrated,measurement of the distance along the probe to the liquid height,plunger height, and so on, can then be determined with very highaccuracy, again by using very low-cost electronic components.

Referring now to FIGS. 30 and 31, two different methods 410, and 412 forcalibration of the probe are illustrated in block diagram. The 410method includes retrieving the system clock calibration factor,measuring the echo time to the end of the elongate measurement probe,calibrate the electronically determined probe length with the clockcalibration factor, calculate the offset “X” between the delay line andthe probe, e.g. the space in which the upper spacer is located, thencalculate and store the calibrated probe length. This method can berepeated as often as needed in the field, and initiated eitherautomatically, such as when a sufficient fluctuation in ambienttemperature occurs, or manually when it is suspected that recalibrationmay be needed. The calculation of the distance X can be accomplished byusing the known physical height of the upper spacer, the calibratedheight based on temperature expansion or contraction of the material, orby measuring the speed of the radar signal through the known spacermaterial and determining the time of propagation therethrough. Thedistance can then be calculated by the determined propagation velocityand the system clock timing.

The method 412 is accomplished, as previously described, by insertingcalibration pins at known locations along the measurement probe,measuring the echo time to each pin, determining the delta time betweenthe pins, and calculating the electronic distance therebetween using thecalibration clock value, then calculating the offset “X” as previouslydescribed. The calibrated probe length can then be stored and retrievedduring actual measurements.

Referring now to FIG. 32, a method 420 for measuring a level ofmaterial, such as liquid, with the TDR measurement system is illustratedin block diagram. The method 420 includes generating the fast and slowcurves, transmitting the radar pulse and receiving return echo data,then determining, using the TVT method described above, whether a returnecho is valid and if so, calculate the difference in time between theupper end of the measurement probe and the top of the material surface,as determined by the valid return echo profile and the maximum slope ofthe TVT curve as previously described, then apply the system clockcalibration factor to calibrate the determined time difference and thusthe actual distance to the top surface of the material being measured.

Referring to FIGS. 36-47, graphs of actual return echo profiles weregenerated using the system and methods of the present invention. Asshown, the TVT curve intersects the return echo profile at distinctpositions on each graph. For a 24-inch probe, it is readily seen thatthe system and methods of the invention enable measurement of the liquidlevel to within 0.490 inches or closer to the top of the upper spacer,the result of which is quite surprising, but is made possible by themechanical structure of the inner electrode, the narrow upper portionthereof, the size and dielectric constant of the upper spacer, whichgreatly reduces the size of any return echo at the transition of theupper spacer and the inner space between the electrodes, since the upperend of the measurement probe is designed to approximate the impedance ofthe empty probe below the upper spacer. Accordingly, the liquid ormaterial level is capable of being measured at a much closer positionwith respect to the top of the measurement probe than prior art devicesrequiring much more expensive solutions.

Also, as shown in FIG. 47, a measurement of 24.1 inches at theintersection of the return echo profile and the TVT curve, demonstratesthat the liquid level or material level can be measured to the lower endof the probe at or near the lower spacer. In this manner, substantiallythe full length of the elongate measurement probe can be used.

The compensation process associated with ensuring accurate liquid levelor linear movement within the elongate measurement probe can be gatheredand processed through known data processing techniques using computeralgorithms or software for various platforms and can be provided ascomputer readable software on various media storage devices fordownloading into and operating on a smartphone, a computer, display, orthe like, including but not limited to hard drives, websites, thumbdrives, flash memory devices, CD's, and so on.

It will be understood that the various measured and calculated valuesassociated with material properties as described above are given by wayof example only and are not intended to be an exhaustive list. Softwaretechniques and methods for accurately determining the liquid level,volume and other tank conditions as discussed above can be implementedin analog circuitry, digital circuitry, in computer hardware, firmware,software, and combinations thereof. The techniques and methods may beimplemented in a computer program product tangibly embodied in amachine-readable storage device for execution by a programmableprocessor; and the above-described methods may be performed by aprogrammable processor executing a program of instructions to performfunctions by operating on input data and generating output. Furtherembodiments may advantageously be implemented in one or more computerprograms that are executable on a programmable system including at leastone programmable processor coupled to receive data and instructions fromand transmit data and instructions to a data storage system, at leastone input device, and at least one output device. Each computer programmay be implemented in a high level procedural or object-orientedprogramming language, or in assembly or machine language, which can becompiled or interpreted. Suitable processors include, by way of example,both general and special purpose microprocessors. Generally, a processorreceives instructions and data from read-only memory and or RAM. Storagedevices suitable for tangibly embodying computer program instructionsand data include all forms of non-volatile memory, including by way ofexample semiconductor memory devices, such as EPROM, EEPROM, and flashmemory devices; magnetic disks such as internal hard disks and removabledisks; magneto-optical disks; and so on. Any of the foregoing may besupplemented by, or incorporated in, specially designed applicationspecific integrated circuits (ASICs).

Although particular embodiments 10, 210 of the TDR measurement systemhave been shown and described, it will be understood that other mountingarrangements as well as other sensing probe configurations can be usedwithout departing from the spirit and scope of the invention. Forexample, a one-inch NPT threaded mounting opening is common on manytypes of holding tanks and therefore it is within the purview of thepresent invention to provide appropriate mounting heads for any tankmounting configuration for connecting the TDR measurement system to thetank wall or other surface.

It will be understood that the term “preferably” as used throughout thespecification refers to one or more exemplary embodiments of theinvention and therefore is not to be interpreted in any limiting sense.

It will be further understood that the term “connect” and itsderivatives refers to two or more parts capable of being attachedtogether either directly or indirectly through one or more intermediatemembers.

In addition, terms of orientation and/or position, such as upper, lower,first, second, inner, outer, vertical, horizontal, and so on, as well astheir derivatives as may be used throughout the specification denoterelative, rather than absolute, orientations and/or positions.

It will be appreciated by those skilled in the art that changes can bemade to the embodiments described above without departing from the broadinventive concept thereof. It will be understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but isintended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

The inventor claims:
 1. A time domain reflectometry (TDR) measurementsystem for determining the location of a boundary between two media,comprising: A. a housing; B. an elongate measurement probe having aproximal end portion connected to the housing and a distal end portion,the elongate measurement probe comprising: a. a hollow outer elongateelectrode connected to the housing and with at least one innerconductive surface; b. an inner elongate electrode located coaxiallywithin the hollow outer elongate electrode, the inner elongate electrodecomprising: i) a first axially extending section with a first lengthwith a first diameter and first outer conductive surface that faces theinner conductive surface of the hollow outer elongate electrode; and,ii) second axially extending section having a second length with asecond diameter and a second outer conductive surface facing the innerconductive surface of the hollow outer elongate electrode, with thefirst diameter being smaller than the second diameter; iii) a firstinner space located between the outer electrode and the first axiallyextending section of the inner electrode; iv) a second inner spacelocated between the outer electrode and the second axially extendingsection of the inner electrode; v) a first spacer located in the firstinner space having a first spacer bore for receiving the first axiallyextending section of the inner elongate electrode, the first spacerbeing constructed of a material with a first dielectric constant that,together with the smaller diameter of the first outer conductive surfaceand at least one inner conductive surface, describe a first impedance;c. the second inner space together with the second outer conductivesurface and at least one inner conductive surface describe a secondimpedance in the absence of the medium to be measured which is equal tothe first impedance, and equal to a third impedance when a mediumoccupies at least a portion of the second inner space; C. an electronicsassembly comprising a transmitter for transmitting a signal along theelongate measurement probe and a receiver for receiving at least areturn echo from the signal upon encountering a change in the impedancewith respect to the inner and outer electrodes to thereby determine theposition of the medium located in the second inner space; wherein thefirst impedance at least approximates the second impedance in theabsence of the material to be measured to thereby reduce or eliminate areturn echo at an interface of the first spacer and the second innerspace so that the material to be measured can be discerned in closeproximity to the first spacer.
 2. A TDR measurement system according toclaim 1, wherein the first spacer further comprises: a first spacer bodywith a first upper spacer surface, a first lower spacer surface, and afirst outer spacer surface that extends between the first upper andlower spacer surfaces and faces the first inner surface of the outerelongate electrode; and the first spacer bore extends through the firstspacer body between the first upper and lower spacer surfaces forreceiving the first axially extending section of the inner elongateelectrode.
 3. A TDR measurement system according to claim 2, wherein thefirst spacer further comprises a first annular groove that extends intoone of the first upper and lower surfaces to modify the first impedanceby combining the first and second dielectric constants.
 4. A TDRmeasurement system according to claim 3, wherein the first spacerfurther comprises: a second annular groove extending into the other ofthe first upper and lower surfaces to further modify the first impedanceby further combining the first dielectric constant with the seconddielectric constant.
 5. A TDR measurement system according to claim 4,wherein the first and second annular grooves are approximately equal indepth and have different diameters.
 6. A TDR measurement systemaccording to claim 5, wherein the first spacer further comprises anouter annular groove in the first outer spacer surface and a firstO-ring located in the outer annular groove for sealing the first spaceragainst the at least one inner conductive surface.
 7. A TDR measurementsystem according to claim 6, wherein the first spacer further comprisesan inner annular groove in the first spacer bore and a second O-ringlocated in the inner annular groove for sealing the first spacer againstthe first outer conductive surface.
 8. A TDR measurement systemaccording to claim 1, wherein the housing comprises a mounting head toconnect to a structure associated with the material to be measured, themounting head comprising: a. a first mounting portion for connection tothe structure; and b. a second mounting portion extending from the firstmounting portion and having an annular side wall with a generallycylindrically shaped electrically conductive inner mounting surface, theouter elongate electrode being mechanically and electrically connectedto the inner mounting surface, the at least one inner conductive surfacecomprising a first inner conductive surface that faces the first outerconductive surface of the inner elongate electrode and a second innerconductive surface that faces the second outer conductive surface of theinner elongate electrode, so that the first inner space is located abovethe outer elongate electrode in the mounting head, the first spacer bodyhaving a first spacer outer diameter approximately equal to an outerdiameter of the outer elongate electrode so that the first spacer islocated above the outer elongate electrode in the mounting head.
 9. ATDR measurement system according to claim 8, wherein the second mountingportion includes an annular flange that circumscribes an opening locatedcentrally in the annular side wall of the second mounting portion andintersects with the first inner annular space; the first inner spacerbeing sandwiched between an upper edge of the outer elongate electrodeand the annular flange to thereby secure the first inner spacer withinthe first inner space.
 10. A TDR measurement system according to claim9, wherein the electronics assembly comprises a printed circuit board(PCB) mechanically and electrically connected to an upper surface of theannular side wall of the second mounting portion so that the outerelectrode is at least electrically connected to the PCB.
 11. A TDRmeasurement system according to claim 10, wherein the inner electrode ismechanically and electrically connected to the PCB.
 12. A TDRmeasurement system according to claim 11, wherein the PCB comprises: afirst trace electrically connected to the outer elongate electrode viathe annular side wall of the second mounting portion; and a second traceelectrically connected to the inner elongate electrode.
 13. A TDRmeasurement system according to claim 12, wherein the electronicsassembly further comprises a processor and timing circuitry associatedwith the processor that can generate a transmit signal and receive atleast a return echo of the transmit signal; a calibration trace on thePCB connected to the second trace, with a physical length of thecalibration trace being known; and an end of trace anomaly to therebycreate an identifiable calibration trace return echo at or near an upperend of the inner electrode for calibrating a clock associated with theprocessor and the timing circuitry.
 14. A TDR measurement systemaccording to claim 13, wherein the end of trace anomaly comprises auser-selectable impedance-changing element.
 15. A TDR measurement systemaccording to claim 14, wherein: the processor activates a plurality oftransmit signals and receives return echo signals during a singlemeasurement cycle for receiving measurement data associated with thetransmit signal for creating a return echo profile based on thecollected measurement data across the predetermined number of transmitand receive signals.
 16. A TDR measurement system according to claim 15,wherein the timing circuitry comprises first analog timing circuitry forreceiving a first timing signal from the processor for actuating aslow-rising function for each transmit signal, a second analog timingcircuitry operably associated with the first analog timing circuitry,and the processor for actuating a fast-rising function, in which areceive signal is generated when the fast-rising function is equal to orgreater than the slow-rising function for receiving data associated withthe transmit signal during each transmit cycle to thereby collect themeasurement data.
 17. A TDR measurement system according to claim 16,wherein the second analog timing circuitry is adapted to automaticallyactivate a transmit signal when the fast-rising function is equal to orgreater than a predetermined value after activation of the receivesignal to thereby activate transmission of the transmit signal along atleast one of the calibration trace and the measurement probe.
 18. A TDRmeasurement system according to claim 17, wherein the fast-risingfunction and slow-rising function comprise non-linear functions ofvoltage verses time.
 19. A TDR measurement system according to claim 18,wherein the non-linear functions comprise exponential functions ofvoltage versus time.
 20. A TDR measurement system according to claim 19,wherein the intersections of the fast-rising function with theslow-rising function over subsequent transmit cycles is expressed by alinear function of voltage versus time.
 21. A TDR measurement systemaccording to claim 20, wherein the slow-rising function is actuated atfirst discrete time intervals during a single measurement cycle and thefast-rising function is actuated at second discrete time intervalsduring the single measurement cycle, the first and second discrete timeintervals being different by at least an order of magnitude such thatthe intersection between the fast-rising function and the slow-risingfunction for each time interval occurs at a different position along theslow-rising function to thereby activate the receive signal at anincreasingly longer time interval for each subsequent actuation andreceive data associated with the transmit signal at an increasinglylonger discrete distance along the measurement probe to thereby gatherdata points corresponding to the predetermined number of receivesignals, the data points being reflective of the transmit signal atleast along the measurement probe at the increasingly longer discretedistances to thereby determine the echo profile of the elongatemeasurement probe and thus the position of the medium to be measuredwith respect to the elongate measurement probe.
 22. A TDR measurementsystem according to claim 21, wherein the first discrete time intervalsare in a range of about 1 to about 999 nanoseconds.
 23. A TDRmeasurement system according to claim 21, wherein the second discretetime intervals are in a range of about 1 to about 999 picoseconds.
 24. ATDR measurement system according to claim 1, and further comprising asecond spacer generally annular in shape and located in the second innerspace between the at least one inner conductive surface of the outerelongate electrode and the second outer conductive surface of the innerelongate electrode at the distal end portions thereof, the second spacerbeing constructed of a material with a third dielectric constant that,together with the at least one inner conductive surface and the secondouter conductive surface, defines a fourth impedance value associatedwith the distal end portion of the measurement probe, the fourthimpedance value being selected from a group of values that: a) isapproximately equal to the first impedance to thereby substantiallyreduce or eliminate an end of probe return echo in the absence ofmaterial to be measured; b) is greater than the first impedance but lessthan the second impedance to thereby create a further return echo withat least a relatively small amplitude signifying an end of the elongatemeasurement probe; c) is approximately equal to the second impedance tothereby substantially reduce or eliminate the further return echo in thepresence of the material to be measured and create the end of probereturn echo in the absence of the material; and d) is greater than thesecond impedance to create the end of probe return echo in the presenceand absence of the material to be measured.
 25. A method for generatinga time delay by comparing the output of a first time-dependentnon-linear function and a second time-dependent non-linear function,such non-linear functions associated in that their non-linearity relatedso that the comparison of their output values produces a time delay thatis a linear function of time.
 26. The method disclosed by claim 25,further used to produce a series of time delays by repeatedly resettingthe first time-dependent non-linear function to a known value andrepeating the comparison between the first and second time-dependentnon-linear functions.
 27. The method disclosed in claim 26 further usedto produce a series of linearly increasing time delays by linearlyincreasing the time between the said reset of one of the time-dependentnon-linear functions.
 28. The method disclosed in claim 26 in which thesecond time-dependent non-linear function is also reset and the seriesof time delays generated is repeated.
 29. The method disclosed in claim26 further employed to develop a profile of an echo signal anddetermining a level of material therefrom using TDR reflectiontechniques.
 30. A TDR measurement system according to claim 21, andfurther comprising a transmit circuit electrically connected to theelectrodes, the transmit circuit being activated for transmitting thetransmit signal when the fast-rising function reaches a predeterminedvalue after intersection with the slow rising function and activation ofthe receive circuit to thereby transmit the transmit signal along theelectrodes.
 31. A TDR measurement system according to claim 21, whereinthe slow-rising function and the fast-rising function are nonlinear. 32.A TDR measurement system according to claim 21, wherein the non-linearfunctions comprise exponential functions of voltage versus time.
 33. ATDR measurement system according to claim 21, wherein multipleintersections of the fast-rising function with the slow-rising functionover multiple transmit cycles create a linear function of voltage versestime to thereby control transmission of the transmit signal andreception of the echo profile at time intervals having very highprecision in the picosecond range.
 34. A TDR measurement systemaccording to claim 21, wherein the electronics assembly furthercomprises a microcomputer having a clock with a pulse output operablyconnected to the first and second analog timing circuits for initiatinga first actuation time of the slow rising function and a secondactuation time of the fast rising function at increasing time intervalsin the nanosecond range.
 35. A TDR measurement system according to claim17, wherein the second analog circuitry activates the transmit circuitat increasing picosecond time intervals for each subsequent actuation ofthe slow and fast rising functions.
 36. A TDR measurement systemaccording to claim 21, and further comprising: a calibration traceelectrically connected between the electrodes and the transmit andreceive circuits, the calibration trace having a predetermined physicallength; processing means for measuring an electrical length of thecalibration trace and correlating the physical length with theelectrical length to thereby calibrate the clock associated with themicrocomputer to a relatively high level of precision.
 37. A TDRmeasurement system according to claim 21, wherein the slow risingfunction comprises a first resistor-capacitor (RC) network, and whereinthe fast rising function comprises a second RC network, with the firstRC network having a relatively higher capacitor value than the capacitorin the second RC network.
 38. A TDR measurement system according toclaim 37, and further comprising a microcomputer that generates a firstclock timing signal at a first output port connected to the first RCnetwork and a second clock timing signal at a second output portconnected to the second RC network to thereby activate the first andsecond RC networks at different time intervals.
 39. A TDR measurementsystem according to claim 21, and further comprising an electronicswitch electrically connected between the microcomputer and the firstand second RC networks for selectively draining and charging the firstand second capacitors of the first and second RC networks.
 40. A TDRmeasurement system according to claim 21, and further comprising a firstcomparator for determining when the second RC network has a value thatis equal to or greater than the first RC network to thereby activate thereceiver circuit.
 41. A TDR measurement system according to claim 21,and further comprising a second comparator for determining when thesecond RC network has a value that is greater than the first RC networkby a predetermined time delay interval to thereby activate a transmitcircuit for transmitting the transmit signal.
 42. A TDR measurementsystem according to claim 41, wherein the slow rising function isactuated a plurality of times during a measurement cycle, with eachsubsequent actuation being delayed by a first duration that increases intime over a previous slow rising function actuation.
 43. A TDRmeasurement system according to claim 42, wherein the fast risingfunction is actuated a plurality of times during the measurement cycle,with each subsequent actuation being delayed by a second duration thatincreases in time over a previous fast rising function actuation so thatthe fast rising function intersects the slow rising function atdifferent positions representative of voltage and time to therebyincrease a duration of the actuated receiver a plurality of times duringthe measurement cycle, with each subsequent actuation being delayed by asecond duration that increases in time over a previous second durationto collect measurement data associated with increasing distances alongthe electrodes.
 44. A TDR measurement system according to claim 21,wherein the slow-rising function is actuated at first discrete timeintervals during a single measurement cycle and the fast-rising functionis actuated at second discrete time intervals during the singlemeasurement cycle, the first and second discrete time intervals beingdifferent by at least an order of magnitude such that the intersectionbetween the fast-rising function and the slow-rising function for eachtime interval occurs at a different position along the slow-risingfunction to thereby activate the receive signal at an increasinglylonger time interval for each subsequent actuation and receive dataassociated with the transmit signal and return echo at an increasinglylonger discrete distance along the electrodes to thereby gather datapoints corresponding to the predetermined number of receive signals, thedata points being reflective of the transmit signal and return echo atleast along the electrodes at the increasingly longer discrete distancesto thereby determine the echo profile of the electrodes and thus theposition of the medium to be measured with respect to the electrodes.45. A TDR measurement system according to claim 44, wherein the firstdiscrete time intervals are in a range of about 1 to about 999nanoseconds.
 46. A TDR measurement system according to claim 45, whereinthe second discrete time intervals are in a range of about 1 to about999 picoseconds.
 47. The method of calibrating a TDR measurement systemincluding: a) applying a known signal to an embedded calibration traceof a known length in the TDR measurement system; b) measuring anapparent length of the embedded calibration trace using the TDRmeasurement system; c) comparing the measured apparent length of theembedded calibration trace to the known length; d) adjusting the TDRmeasurement system's clock so that the measured apparent length iscorrected to the known length; e) maintaining the TDR measurementsystem's adjusted clock.
 48. The method of claim 47, including theadditional step of repeating the previous described steps periodically.49. The method of claim 47, comprising additional following steps tocorrect for temperature variation: a) applying a known signal to anembedded calibration trace of a known length with a knowntemperature-versus-length profile stored in the TDR measurement system;b) measuring the ambient temperature of the TDR measurement system; c)correcting the known length of the embedded calibration trace tocalculate a temperature-compensated length; d) measuring the apparentlength of the embedded calibration trace using the TDR measurementsystem; e) comparing the measured apparent length of the embeddedcalibration trace to the temperature-compensated length; f) adjustingthe TDR measurement system's clock so that the measured apparent lengthis corrected to the temperature-compensated length; g) maintaining theTDR measurement system's adjusted clock.
 50. A TDR measurement systemaccording to claim 13, wherein the end of trace anomaly comprises abiased diode, switch, transistor, inductor, capacitor, or resister,which can be selectively unbiased during calibration and biased duringmeasurement to create the calibrate trace return echo.
 51. A TDRmeasurement system according to claim 1 in which one or more electrodesis constructed with an insulative coating on its conductive surface. 52.A TDR measurement system according to claim 51 in which one or moreelectrodes is constructed with an insulative coating on its conductivesurface, said coating applied to tune the impedance of said electrode.53. A TDR measurement system according to claim 51 in which one or moreelectrodes is constructed with an insulative coating on its conductivesurface, said coating applied to protect the electrode from corrosion.54. The method of calibrating a TDR measurement system for first use orservice, comprising: a. measuring a known material level with the TDRmeasurement system while at a known stable ambient temperature to obtaina measured material level; b. comparing the measured material level tothe known material level; c. calibrate the TDR measurement system sothat the measured material level is equal to the known material level;d. measuring the apparent length at the ambient temperature of anembedded calibration trace with a known temperature-versus-lengthprofile; e. calculating the default length of the embedded calibrationtrace at a user-defined default operating temperature based on theapparent length measured at the known ambient temperature and thetemperature-versus-length profile of the embedded calibration trace; f.storing the default length for later use in measuring materials at anunknown level and varying temperatures.
 55. The TDR measurement systemas described in claim 13 with multiple impedance changes deliberatelyconstructed on the electrode or calibration trace.